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Imagine a single, long, straight stretch of track. On the track there is a train heading in one direction at 100 km/h and a second train heading towards it at 200 km/h. The latter train is a special priority train that should not slow down at any cost.

Edit: Let us assume the trains are 75 km apart, resulting in 15 minutes until a full-on collision if no action is taken. Answers needn't adhere to this guideline.

How do I prevent the former train from slowing down the latter without causing major damage to the former train?

Answers adjusting the infrastructure or train construction before the encounter are acceptable, so long as there are no fixed bypasses or signals.

Reasons why such a long, single track may exist could, for example, be:

  • The line is temporary or new and only a single track has been completed.
  • There are spacial limitations such as the line running through a narrow valley/ravine
  • There are structural limitations such as the surrounding ground being highly expensive to stabilise sufficiently
  • There are political or cultural limitations such as funding cuts, building permissions, heritage bridges/tunnels or surrounding nature reserves
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    $\begingroup$ "Although accidental derailment is damaging to equipment and track, and requires considerable time and expense to remedy, derails are used in situations where there is a risk of greater damage to equipment, injury or death if equipment is allowed to proceed past the derail point." (A "derail" or "derailer" is a device used to prevent fouling -- blocking or compromising -- of a rail track, or to avoid collisions with anything present on the track; it works by derailing equipment passing over it.) I don't understand what a "safe" derailment would be. $\endgroup$ – AlexP Mar 27 at 9:36
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    $\begingroup$ The fastest solution - the one that avoids slowing down the priority train - is stopping the other train, then force it to go backwards until the closest place where it can move out of the way (a railroad switch) or until the priority train destination. Any other solution is going to take even more time. $\endgroup$ – Rekesoft Mar 27 at 11:55
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    $\begingroup$ @Rekesoft Having the opposing train become part of the priority train? That would be a sensible answer I'd be willing to upvote. $\endgroup$ – A Lambent Eye Mar 27 at 11:59
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    $\begingroup$ @UKMonkey Isn't that what makes it worth being a question? The fact that bypasses aren't permitted? $\endgroup$ – A Lambent Eye Mar 27 at 16:25
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    $\begingroup$ This sounds like the plot of a movie. "High priority train"? More like "there's a bomb onboard that will detonate if the speed goes below 200 km/h". $\endgroup$ – The Great Duck Mar 28 at 3:13

22 Answers 22

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The Fast and the Furiously Crazy

Since you've eliminated the sensible solution (bypasses or double tracks), let's go with an insane one!

All your trains have rail tracks running on top of them and extra wagons with ramps at the front and back. When a priority train approaches, they lower the ramps onto the rails and the priority train drives straight over them as if it were a bridge.

Some notes:

  • The slower train should drive at maximum speed if overtaken from behind or stop if the prio train approaches from the front.
  • The ramps will need to be very long to prevent the prio train from jumping the tracks or going completely airborne. Rollercoaster-like guardrails could assist here, adding the weight of the bottom train to the downward force.
  • This only works with very straight tracks. Very. Straight.
  • It is not recommended to attempt overtaking a train that is overtaking another.

A more boring but slightly more sensible variant of this is that the slower train stops at a depression/valley in the track so that the rails on top become level. You could even have moving sections of track that can lower so that the slow train when stopped there is essentially replacing that section with its roof rails. This might count as a fixed bypass though.

Sideways treads

Here is a second more boring solution: All locomotives and wagons are powered and carry retractable treads underneath that allow sideways movement. These treads are wide and solid to support the train, but have indentations to protect the rail bars from damage.

When a train is commanded to make way it comes to a stop, lowers the treads to the ground and moves to the side until it is clear of the tracks. After the prio train has passed, it rolls back onto the track, aligning the wheels with the rails carefully, then retracts the treads and resumes its journey on the rails.

In these trains, each wagon has its own electric motors for driving both wheels and treads. This increases the total weight of the train, but distributes it better than pure locomotive/unpowered wagons so there is no 250 ton locomotive to move onto the mud. The locomotive in this case mostly houses the (diesel) generators that supply electricity to the train and the controls.

The ground next to the train tracks needs to be level and sturdy enough to support the weight of the train, but not quite to the standard of the rails themselves.

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    $\begingroup$ Since I'm going to be the duty rules lawyer on this, maximum train climb gradients, trains really don't like hills. I will ignore any issues about overloading the lower train structure, axles, or the tracks. $\endgroup$ – Separatrix Mar 27 at 10:05
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    $\begingroup$ A similar solution to my first thought, but then you also get problems with the front of the train moving at a different speed to the back when climbing over the other train, unless the lower train is absolutely still. The problem I see with the depression is that the train needs to be of a specified length to 'fill the gap'. $\endgroup$ – A Lambent Eye Mar 27 at 10:40
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    $\begingroup$ @Separatrix trains are about 4 - 4.5m high and carriages ~20m long. If the ramp is two carriages long, that is 4:40 or a 1.10 incline, which is below various of the inclines given in the linked article. So it certainly could work if the overtaken taken is at least 4 carriages long, to be able to carry 2 such ramps. $\endgroup$ – Polygnome Mar 27 at 12:45
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    $\begingroup$ You mean like this time it was done in real life but with longer rails? $\endgroup$ – Joe Bloggs Mar 27 at 20:38
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    $\begingroup$ @JoeBloggs wow that is just amazing! So yes, exactly that, except at 100 km/h speed difference >:-) $\endgroup$ – Cyrus Mar 27 at 21:14
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Two trains become one.

On the track there is a train heading in one direction at 100 km/h and a second train heading towards it at 200 km/h.

You do not specify the distance between them. If there is some distance there is time for this maneuver.

  1. Slow train slows down, stops, goes into reverse.

  2. Previously slow train accelerates, in reverse, until it is going almost 200 km/h.

  3. Fast train will slowly close the distance. When the two trains are very close, they are linked. This is not something routinely done with fast moving trains but is done all the time with slow moving trains. It does not seem outrageous; relative to one another the trains are barely moving, or even not moving. It is akin to refuelling a plane in flight except easier because the trains are on the same track.

  4. You now have one fast train, going the specified direction at the specified speed. The fast train did not have to break stride. The slow train did not leave the tracks. You did not have to build anything new.

The neat thing about trains is that the size of the train is fluid - it can be longer or shorter according to need, and trains can be merged and split.

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    $\begingroup$ You could even trasfer things from one side of the train to the other and have the other basically detach and you have now the slow train again. Lovely solution and reminds me of the replacing a ship part by part paradox :) $\endgroup$ – Hakaishin Mar 27 at 18:18
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    $\begingroup$ @Hakaishin - I like that very much, and would like it even more if I were a passenger on the slow train. $\endgroup$ – Willk Mar 27 at 18:22
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    $\begingroup$ A related but likely simpler solution: the train stops, then accelerates in the opposite direction to 201km/h. Sort out this snafu after the trains reach the destination. $\endgroup$ – Cort Ammon Mar 27 at 19:13
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    $\begingroup$ @CortAmmon - “WORK? Why, cert’nly it would work, like rats a-fighting. But it’s too blame’ simple; there ain’t nothing TO it. What’s the good of a plan that ain’t no more trouble than that? It’s as mild as goose-milk. Why, Huck, it wouldn’t make no more talk than breaking into a soap factory.” Tom Sawyer, from Huckleberry Finn. $\endgroup$ – Willk Mar 27 at 19:43
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    $\begingroup$ The biggest problem with this is that I wouldn't expect the regular everyday trains to be able to reach the same speeds as the priority train (otherwise they'd already be going that speed), which means that the priority train will have to put work into pushing the other train along, and will have to slow slightly. Exact amount of slowing depends on the relative masses and engines of the two trains, of course $\endgroup$ – Arcanist Lupus Mar 28 at 6:15
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Jackscrews

The slow train will come to a complete stop. At both ends of each of its cars, two outrigger hydraulic- or screw jacks (each as tall as the train) are extended outwards to beyond the fast train's width, and after this downwards to beyond the height of the fast train, lifting up the entire slow train. Effectively, this forms a tunnel underneath the slow train through which the fast train can travel.

After the fast train has passed, the train will be lowered exactly to its original position. If necessary, minor corrections can be performed laterally by varying the outrigger's position.

For those who believe one cannot lift a heavy train with compact jack screws or hydraulic jacks, look no further than self driving cranes.

Apparently, a prototype already exists:

(both images from Wikimedia)

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I have an idea which I'll try to put into writing but it might not be obvious what I'm driving at. I actually had a couple of thoughts on this but one may be more sensible than the other.

Up and Over

My initial thought was that one of the trains, probably the 200 km/h one as it would already likely be a streamliner, would be designed in such a way that it's front is like a ramp and it has rails built into the ramp which run along the track in front of it. These rails continue over the carriage roof and the rear of the train look like the front. The oncoming train could then be forced to run over the top of the fast train. This does however require that the train that goes over is able to climb an unrealistic gradient although oncoming speed may assist. It also assumed that there are no overhead electricity cables but that the train above is able to continue to proceed without a "third rail" either.

Shall we dance?

As an alternative to the up and over method, I came up with something that could actually work. On a traditional track, you'd place the 100 km/h train in a siding and have it wait until the 200 km/h train had passed. I realize that this can't work because it requires you to know where the siding would need to be.

However, when you consider what a train requires, it is essentially rails on which to run. Now, imagine that each train is carrying some sort of short section of rail at its front which is angled from the right to the left of the track. When the two trains come close enough, these angled tracks collide and are forced into the rail bed. This causes the left-hand wheel set of each train to jump off the left hand track and the right hand wheel set is forced onto the left hand rail. This effectively derails both trains simultaneously causing a massive accident. However, if the tops of each train were designed to carry some sort of rail / tube on top of the train with some sort of interlocking arm, the weight of each train would be carried by the other.

Each train would hold the other up, a little like a spinning ballerina is supported by one of two feet and by a dance partner at the top of their arm, outstretched above their heads. Both locomotives would progress along the same piece of track but using just one rail each, each locomotive offset and supported by the other.

Once they had passed each other, an assembly at the rear of each train could "re-rail" the wheel sets back to their original location which would effectively by a mirror of the assembly at the front of the train.

Hopefully this makes some sense. If not, I could possibly try to sketch out how it would look.

Edit Added sketch

sketch

Edit 2 — "Budge Over" trains

You could potentially re-design the trains to allow the "shall we dance" method to be a little less severe. Instead of forcing the trains to jump to the opposite track, design the trains to have an angled cab with looks like a triangle when viewed from above. Along one side of the train, have interlocking "rails" which would interface with each other, shoving the oncoming train onto the side of the opposing carriage.

Redesign the wheel sets so that they had one fixed set of wheels and one "sprung" set which were floating so the trains could continue to run one wheelset on a single rail but the other would float in free air under the train.

This would effectively allow trains to "slither" past each other. Whilst this refinement to the design probably makes the solution a little more realistic, it does remove some of the grand drama that the "Shall we dance" method has.

another sketch

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    $\begingroup$ What an elegant and adrenaline-filled solution! A sketch would nevertheless be appreciated, especially since I am unsure as to how the re-railing would work. There also may be difficulties if the trains have two vastly different lengths. $\endgroup$ – A Lambent Eye Mar 27 at 11:46
  • $\begingroup$ I'll knock a sketch together. I did wonder if different length trains could be catered for by the mechanism on the roof having some sort of extending retractable pole / cable that is only released at the point the longer train is back on both rails. Perhaps it could spring back at high speed like a massive version of one of those retractile steel tape measured being released? $\endgroup$ – Steve Matthews Mar 27 at 11:50
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    $\begingroup$ I'd have visions of the mechanism on the top of the train initially looking a little like a jousting knight on top of a horse. You'd effectively be using the weight and momentum of each of the trains to be the counterweight of the other. $\endgroup$ – Steve Matthews Mar 27 at 11:59
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    $\begingroup$ It's brilliant, it could never work, but who cares. $\endgroup$ – Separatrix Mar 27 at 12:06
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    $\begingroup$ Also: A train is about 3m wide, so they will need to be displaced by at least as much (could be lessend if both trains 'balance' on the rails on a middle wheel, steadying each other, instead of 'hanging' from the rails on their side wheels and each other). If they pass each other at a relative 300km/h (84 m/s), and have 8 meter long mobile switches in front, they would need to be displaced by 3m in 1/10th of a second (and not have any lateral velocity left at the end) They would pull +120, then -120 g lateral. Quite a jolt. Love it. $\endgroup$ – bukwyrm Mar 27 at 14:33
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You can't

Well you can, but it's going to take a few days. First you're going to need to get the infrastructure in place.

A train weighs between 1500 and 6000tons. I'm assuming this is a passenger train rather than a cargo train which could weigh nearly 100,000 tons.

We're going to need cranes in place able to lift at least 250tons just to move the engine out of the way. Such things do exist as they're part of the breakdown and derailment recovery processes but they're not exactly common, it may take a day or two just to get them into place.

You also need to make sure there's a safe and stable surface to put your engine onto that isn't the tracks. It still weighs up to 250tons, so you can't just put it down on unprepared ground and preparing ground for that sort of load takes time.

I'm sure you can see where this is going. There's no quick and safe way to remove a train from the tracks.

Accidental derailment also damages the tracks often over long distances, a general theme is that it takes a week to ten days to recover and repair after a relatively minor incident.

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  • $\begingroup$ Could there be a way of modifying the track or train wheel morphology to allow the train to be moved quicker? The thought occurs since one would at least need to clear the flange from the rail. $\endgroup$ – A Lambent Eye Mar 27 at 10:44
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    $\begingroup$ @ALambentEye, the flange is only a couple of inches, the engineering of train wheels is beautiful in its simplicity. The issue is simply the sheer weight of what you're trying to move in a safe manner. $\endgroup$ – Separatrix Mar 27 at 10:59
  • $\begingroup$ Could there be a way of constructing the locomotive and carriages to 'eject' everything but the actual frame off the track and onto a deployed surface to distribute weight before having the frame lifted off? I could imagine that could speed up the process. $\endgroup$ – A Lambent Eye Mar 27 at 11:36
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    $\begingroup$ @ALambentEye, let's say you build the tracks on a wide base so there's suitably prepared ground either side of the tracks, you then build all your trains with side struts like a crane, these can be extended and used to move the entire train up and across to clear the tracks. You then have to justify why you've done something so complex when it would be quicker, cheaper, simpler, and less maintenance to build parallel tracks. $\endgroup$ – Separatrix Mar 27 at 11:46
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    $\begingroup$ @ALambentEye Under usual circumstances only a single rail is required If you need to fit every train with a Rube Goldberg Device in case a super special train comes along, you definitely have enough circumstances justifying another set of tracks or two. $\endgroup$ – AmiralPatate Mar 27 at 15:15
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Use road wheels on the slower train. Build it out of railcars like these:

road-rail vehicle road-rail vehicle

When the faster train approaches, just stop, raise your rail wheels, and drive off the tracks.

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    $\begingroup$ Could you elaborate on how that could help? I imagine it being rather difficult to get on and off the tracks without making use of a road crossing. $\endgroup$ – A Lambent Eye Mar 27 at 13:35
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    $\begingroup$ @ALambentEye They make plastic ramps exactly for that purpose. The problem with this solution though is that logically, if there was space to take the slow train off rail, you might as well install a siding there. $\endgroup$ – user71659 Mar 27 at 15:00
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    $\begingroup$ @ALambentEye : The Unimog has amazing off-road abilities, and it can be equipped with rail wheels. Unfortunately I couldn't find a video showing both. But the very same vehicle which you see climbing in that video has variants equipped with rail wheels. $\endgroup$ – vsz Mar 27 at 16:16
  • $\begingroup$ How would this work with carriages? $\endgroup$ – A Lambent Eye Mar 27 at 16:20
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    $\begingroup$ @ALambentEye : Does the question require only using existing railcars without any modifications? If not, then equip your carriages with road wheels too... or just couple several Unimogs together to form your train. Or just make a custom designed vehicle, based on this idea. I only showed that there are already vehicles which can travel on rail and also have good off-road capabilities. $\endgroup$ – vsz Mar 27 at 16:25
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On the slow train by each set of wheels also put a set of 3 wheels at 90 degrees to the main ones, each on an extending pistons.

When it needs to let the fast train pass it stops and lowers the wheels on pistons, then drives sideways off the track (it needs 3 so it can lift one to pass over the rails and still have 2 on the ground).

Once the fast train has passed it reverses the process to drive back onto the train and recenter itself then lifts the extra wheels and continues.

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Depending on your technology level, all train tracks could be raised off the ground and have rails above and below. All East/North bound trains run on the top rails. All West/South bound trains travel hanging on the bottom side of the tracks. Inside the cars are tubes so that the ceiling can always remain up. Every 10 kilometers or so, you could have loops that could move a train from one track to another, so a faster train could overtake and pass a slower train going the same direction.

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  • $\begingroup$ I think this violates the "no fixed bypasses" clause of the question. Really, it's just a complicated version of "Build two tracks instead of one, and put them on top of each other instead of side-by-side." $\endgroup$ – David Richerby Mar 28 at 13:08
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Your only real hope is a track gang who can move fast.

They find a place along the track near the starting position of the slow train, where a bolted joint exists that's staggered by just a few feet between the two rails. It should not be a special joint such as an insulated joint. If it's not there, get a rail saw and rail drill in advance, and create a joint. The crew stages out there with a bulldozer and a number of semiloads of track panels and a crane, and they lay down enough temporary track to easily fit the slow train.

Long before it arrives, they unbolt the main track, shove one main about 5 feet to the left, the other 5 feet to the right (the staggering decides which goes which way), grade the subgrade to level, and drop in the temporary track to meet it. Throw 2 bolts in the joint bars, no more. Have the slow train crawl into this temporary track, and about 100' past the joint.

Now you have 15 minutes to reverse. A bulldozer is already chained to the temporary track, and six other bulldozer or big SUV winches are tied to the main track, ready to pull the segments back where they belong. Yank the 2 bolts, pull the tracks over, and a few workers tighten the 6 mainline bolts while many other workers with gas powered jackhammers tamp the main track back to level. ZOOM, the other train tears through.

This is achievable with a crew that knows what it's doing. Railroad track is "lego" like that.

Rinse, wash, repeat to back the first train out onto the main again, reassemble the main, and the first train is on its way.

Of course you know, nobody goes 200kph without some sort of automatic signal systems to prevent collisions. There'll never be any danger of collision, because the signal system will stop the trains if a train is in the way or the track is severed.

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    $\begingroup$ Until that darn engineer named Casey Jones starts doing lines of coke ... or someone gets on the wrong track and is headed for you... $\endgroup$ – ivanivan Mar 28 at 3:11
  • $\begingroup$ moving the track? nope. nope nope. These things (should) have foundations. You aint moving that and replacing it any time soon. $\endgroup$ – Baldrickk Mar 29 at 9:50
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    $\begingroup$ @Baldrickk Normal railroad track is rails and ties sitting in cut stone (that tends to interlock). There is a good subgrade under the stone, but that only matters on mainlines. Here you need to grade a rough subgrade, that's why you have a bulldozer. Slap bare track down on that. At that point, it's fit for the ballast train, which is a heavy train. You absolutely can bend track around like this, it's called a "cutover". 15 minutes is a little quick, normally they take an hour or two. Not much longer than that, it costs a fortune to have a mainline closed. $\endgroup$ – Harper Mar 29 at 15:51
  • $\begingroup$ Really, an SUV crashing into a trackbed can knock it out of alignment. $\endgroup$ – Harper Mar 29 at 15:57
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Hold the regular train at the last switch or junction

The question says:

Answers adjusting the infrastructure or train construction before the encounter are acceptable, so long as there are no fixed bypasses or signals.

Rail roads always have sidings and some form of communications specifically to keep trains from arguing over right-of-way while at mutual approach, better phrased as "colliding". Therefore, by construction of the question, at each end of this long section of single track there is a switch for double tracking, siding, or some other place where a train could wait.

A high priority train in the absence of traffic signalling should run on a timetable. The rail traffic control point at each end of the long section of track will have the latest timetable for when the high priority train is scheduled to enter and to leave that section. They will therefore hold all oncoming traffic until they observe that the high priority train has departed the section.

The rail road company would probably make a major effort to improve signalling between the control points, so that they can communicate when a train enters the section and when it leaves. This might be beyond the scope of your work, however.

This infrastructure isn't only for the benefit of the high priority train -- this is for the benefit of all trains, and maintenance as well: Any track work or blockage has to be handled without additional trains making the situation more blocked.

Edit based on comment: Even for an unscheduled express or emergency train -- especially for an emergency train -- the rail road will have some form of communication specifically to prevent unintended cases of head-on collisions.

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    $\begingroup$ Well yes, but no. The priority transport could, for example, be an emergency service. In this case it would not make sense to have the train bond to a timetable. $\endgroup$ – A Lambent Eye Mar 27 at 15:20
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    $\begingroup$ Good point, I hadn't thought of that. And still, there WILL be some way to communicate ahead, "HOLD ALL TRAINS". $\endgroup$ – Codes with Hammer Mar 27 at 15:24
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    $\begingroup$ It seems to me that a civilization capable of building a single rail block 75km long fully rated for trains traveling 200 kph could also figure out the logistics to clear the track when the super-express needs to use it. $\endgroup$ – Codes with Hammer Mar 27 at 15:34
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    $\begingroup$ That is precisely what we are trying to figure out. $\endgroup$ – A Lambent Eye Mar 27 at 15:39
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    $\begingroup$ Simply put, if you have a track for a train which absolutely, positively CANNOT be stopped or delayed, and for which the schedule is not predictable, you don't allow any other traffic on that track unless it can be diverted from that track safely with minimal notice. $\endgroup$ – Theo Brinkman Mar 28 at 20:27
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No problem. Captain Kirk is orbiting above the trains in the Starship Enterprise. He has Scotty take over the transporter and Scotty beams the first train behind the second to avoid the collision.

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    $\begingroup$ Why didn't I think of this before? Ingenious! $\endgroup$ – A Lambent Eye Mar 27 at 20:02
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    $\begingroup$ @ALambentEye -- Sort of begs the question -- With such transporter technology, why are trains still being used? $\endgroup$ – MaxW Mar 27 at 20:05
  • $\begingroup$ @MaxW perhaps the operating cost of the transporter far exceeds that of trains, and so it's only used in special circumstances (like letting a high-speed emergency train continue unfettered while beaming the slower train past it)? $\endgroup$ – Doktor J Mar 28 at 17:48
  • $\begingroup$ In that case, you might as well just teleport fast train directly to it's destination, instead of just past the other train.. $\endgroup$ – GPS Mar 29 at 9:56
  • $\begingroup$ Or maybe transporter cost scales with distance, so teleporting it a few meters to get it past the train is cheaper than teleporting it far. $\endgroup$ – Daniel Vestøl Mar 29 at 14:05
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Portals

Simple, place a blue portal in front of the fast train and an orange portal behind the slow train. Collision averted.

Of course, if you've got portal technology the need for trains might be a bit moot.

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    $\begingroup$ Operation of said portals might require some amount of terribly expensive Unobtanium that renders them unsuitable for daily transport of goods and people. Using them to divert a non-priority train out of the way of an emergency priority train though might justify the cost of said Unobtanium. Actually deploying the portals though might be its own separate logistics issue! $\endgroup$ – Doktor J Mar 28 at 18:04
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    $\begingroup$ @DoktorJ Nah, just use another train... $\endgroup$ – aslum Mar 28 at 18:38
  • $\begingroup$ If you have portals just have one at the source and the destination, and then you only need a few thousand feet of train track on each side. Don't wait till they get close to each other. $\endgroup$ – cybernard Mar 28 at 19:31
  • $\begingroup$ Yeah, I suppose if you have an emergency train, why not just open the blue portal at its origin, and open the corresponding orange portal at the destination? It'll get there much, much faster than traveling by rail at 200kph, and the 100kph train won't even have to think about dealing with it. $\endgroup$ – Doktor J Mar 28 at 21:31
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An impractical but possible 2nd answer.

The fast train has lifts at both ends and tracks on the roof. The slow train stops, reverses, and allows fast train to catch up. Fast train now uses front lift to raise carriage, roll it across the top of itself and put it back down with the opposite lift. Does this with all carriages and engine then the 2 seperate and the slow train stops, reverses, continues.

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  • $\begingroup$ Is this a more compact version of a ramp, as per Cyrus' suggestion? $\endgroup$ – A Lambent Eye Mar 27 at 16:01
  • $\begingroup$ yep I guess it is a variation on the same theme $\endgroup$ – guest Mar 27 at 16:03
  • $\begingroup$ Difference between this and a ramp is that it's (slightly) more feasible. Trains don't do ramps, but they can do lifts. And doing this carriage by carriage works, and takes advantage of the modular construction of trains. If max speed of slow train is 100km/h, this is better than the "just couple both trains together" option, as it allows the fast train to speed up again after they meet, minimizing time lost. $\endgroup$ – Dewi Morgan Mar 27 at 16:47
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Catch Points

Catch Points exist explicitly for this scenario, to redirect a runaway train off a track or away from a dangerous situation without requiring it to be slowed down.

If you need a nice ad-hoc solution using no specific rail safety measures, perhaps a dead-end track-switch or branch that never went anywhere would work in place of a purpose-built catch-point.
Just remove the end-cap/bumper that blocks the track, switch the low-priority train onto that, drive straight off the end of the track and coast on solid ground.

Once the low-priority train is out of the way, switch the track back and watch the high-priority train rush past safely.

These dead-ends can be found in a lot of places, often in places where a train track has been torn up due to age or redundancy. They might be in documentation somewhere, but narratively it's the sort of information an expert on the area would have and someone who just travels the line wouldn't notice.

This approach would be great for dramatic effect.

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    $\begingroup$ You managed to find the corner case I was expecting: Not a derailer, so no damage, but also not a bypass, since it doesn't rejoin. I suppose my only argument would be that they would have to be placed regularly since one may not know whether one can reach the next before the priority train is confronted. $\endgroup$ – A Lambent Eye Mar 27 at 16:11
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    $\begingroup$ Very true, which is why I suggested a dead end of an unfinished or partially dismantled line as being useful as an ad-hoc Catch-point. I'm particularly liking that using this would require lateral thinking and area-knowledge from any characters in your story or narrative. So it's dramatically much more satisfying than "and they used a well placed safety measure to safely derail a train" $\endgroup$ – Ruadhan Mar 27 at 16:16
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    $\begingroup$ It is a classic point of drama in railway novels, or so I'm told... $\endgroup$ – A Lambent Eye Mar 27 at 16:19
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    $\begingroup$ Catch points derail the train. Derailling a train at 100km/h is absolutely not safe for the occupants of that train. It's safer than colliding with a train going at 200km/h in the opposite direction, but you're still going to have fatalities. Specifically, running off the track will cause the front of the train to decelerate suddenly, while the back continues to run forwards and concertinas. Now you have a selection of railway carriages at 90 degrees across the track, ready for the fast train to plough into. $\endgroup$ – David Richerby Mar 27 at 16:50
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    $\begingroup$ The takeaway I'm getting on this is that length of train matters, as does the length of the dead-end/catch-point. So if Lambent Eye wants to use this, it's just a matter of nudging the numbers until they fit. $\endgroup$ – Ruadhan Mar 29 at 9:24
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Use a large Outrigger Suspension system

If you google "Outrigger on crane" you will find many examples of retractable rigging on smaller vehicles to prevent them from tipping over, usually cranes. The trains in your world could all be fitted with a variant of this system that is designed to hydraulically lift the cars of the train high enough to allow the other one to pass beneath.

The contact points would all be well outside of the track and it would not matter much the location at which the train stops since each contact point can be raised to a variable height. The best part is that it would only take a couple minutes to fully suspend the train. Also, since all trains could theoretically be outfitted with something like this, shifting priorities in the train schedule would have little impact.

Furthermore, the use of this technology could be justified by explaining that the cost of implementing something like this on all trains is cheaper that laying a completely separate parallel track.

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    $\begingroup$ Not to mention, if parallel tracks are impossible due to terrain, right-of-way, etc, this only needs about 1m to either side of the train (2m total) for the rising train to plant its outrigger lifts, as opposed to ~4m to accommodate two trains side-by-side with a safety margin. $\endgroup$ – Doktor J Mar 28 at 17:46
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Rockets

Given that there are long stretches of track that the high-priority train may be traveling on, the design of the high-priority train must take into account this scenario. Ideally, it is able to handle these situations itself, as the oncoming slow train might not have the means to get out of the way, the slow train's mechanisms might fail or be maintained to a lower standard than the high-priority train's operation safely permits, etc., and the slow train might not be capable of match the high-priority train's speed for coupling manoeuvres.

The high-priority train can take care of the situation itself if it elevates itself above the track for a brief period of time. Rockets can produce enough thrust to lift a train, although each engine and carriage will need rockets attached, and will need enough fuel to fire twice - once to lift, and once to land. This will be a significant portion of the train's weight. The force will need to be great enough to lift the train for long enough to pass the underlying train; at a relative speed of 300km/h it will take 24 seconds for both trains to pass each other, and (disclaimer: I don't know rocket physics) a set of rockets with ~450kN thrust should be able to lift the train to around 250m for a safe passage, and the fuel for this will be an extra 40 tons. You'll probably need more fuel (and slightly more thrust) in case the high-priority train needs to do this multiple times, the trains' combined length is longer than the 2km I used, and to ensure you have impulse to spare when landing to ensure you can line up with the tracks again accurately. These numbers are for a 250-ton engine; less-powerful rockets will be needed for the carriages depending on their weight.

Rocket engines of this size are similar to the Merlin engines in the Falcon; depending on the number of carriages the train has, it might have a comparable amount of thrust to the complete set of engines in a Falcon-9. It still won't go very high though, because the train is aerodynamically terrible going in a direction it's not meant for, and it won't be carrying much rocket fuel.

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  • $\begingroup$ Although I myself am also not an expert in rockets, I believe them to be rather massive and unstable if constructed or calibrated incorrectly. Anyhow, points for creativity! $\endgroup$ – A Lambent Eye Mar 28 at 9:11
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Add a tilting mechanism to the carriage wheels and the side of the slow train, allowing it to stop, safely tilt itself off the side of the track, and then re-stand itself onto the track after the high-priority train passes safely.

Step 1: The train stops. Step 2: The train extends the side mechanism all the way to the ground. Step 3: The train extends the wheel-side mechanism while retracting the side mechanism, such that it rotates 90 degrees off the track onto its side. Step 4: The train retracts the wheel-side mechanism, leaving the train laying safely on its side. Step 5: The train uses the side-mechanism's articulation to pull itself a safe distance from the tracks so it out of the way of the priority express.

To re-rail the train, simply reverse the process.

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  • $\begingroup$ One must also consider the fact that cargo and passagers inside the train may have difficulty with the change in orientation. $\endgroup$ – A Lambent Eye Mar 29 at 7:30
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    $\begingroup$ Cargo could be dealt with by anchoring it in place when it is loaded. Passenger cars could simply have an internal cabin which rotates freely (but damped, to avoid motion sickness) within the car, so that they never face any direction but up. Or passengers could be debarked before the maneuver takes place. $\endgroup$ – Theo Brinkman Apr 1 at 18:36
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The second train has a lower area for the first train to pass through. enter image description here You would have to use belts,gears and chain, or something to connect the second trains engine to the wheels.

Either that or the second train is 100% electric and multiple train cars have motors to spread out the work load.

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  • $\begingroup$ This could make the higher train less stable due to the higher center of mass. $\endgroup$ – A Lambent Eye Mar 29 at 7:32
  • $\begingroup$ @ALambentEye I am sure some engineer, way smart than me(not a engineer), can figure it out. $\endgroup$ – cybernard Mar 29 at 11:41
  • $\begingroup$ I will mention that pulleys are used for lifting things - not very good for circular motion. In addition, basically all trains are electric, even the diesel trains. This is because diesel engines don't provide enough torque at low speeds. $\endgroup$ – Daniel Vestøl Mar 29 at 14:03
  • $\begingroup$ @DanielVestøl your right I meant belts, like they do in cars, or something else more sturdy. Deleted pulleys cause that was just completely wrong. $\endgroup$ – cybernard Mar 29 at 14:47
  • $\begingroup$ So basically, this thing, only for trains? $\endgroup$ – Darrel Hoffman Mar 29 at 20:01
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The most realistic way to do it fast, if you don't have the infrastructure in place, is to:

  1. Stop the less prioritary train;
  2. Use a crane to move it off the rails;
  3. Once the priority train has passed, use the crane to move the less prioritary train back onto the rails.

Cranes are awesomely strong. Just go to Google and do a dearch image for 'crane 100 tons'. You'll see that a lot.of relatively small models can lift that much. Also remember that even if your train weights much more than that, you only have to lift one wagon at a time.

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    $\begingroup$ A 100ton crane can lift up to 18 tons and you probably need to lift well over 200tons to move the engine. The numbers for commercial and industrial vehicles are a bit weird until you get used to them. A simple example is that a 3.5 ton van has a cargo capacity of around 1.5 tons because the other 2 tons is the van itself. $\endgroup$ – Separatrix Mar 27 at 10:38
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    $\begingroup$ @Separatrix "A 100ton crane can lift up to 18 tons" I'm not sure where you're getting that information from. For example, the Terex AC100/4 is described as a "100 ton crane" and it can lift 100 tons (see spec sheet, p9) at zero radius. More realistically, it can lift 80 tons at a radius of 3m. However, I doubt that one could lift even an 80t locomotive (perfectly reasonable for a European passenger locomotive) off the tracks in just 15 minutes: that takes a lot of preparation. $\endgroup$ – David Richerby Mar 27 at 16:13
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    $\begingroup$ @DavidRicherby I was reading data sheets from one of the manufacturers along with generic search results. $\endgroup$ – Separatrix Mar 27 at 16:37
  • $\begingroup$ @Separatrix Ah. Perhaps the data was for lifting at some "standard" radius or something? $\endgroup$ – David Richerby Mar 27 at 16:41
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    $\begingroup$ @DavidRicherby potentially, since the lifting capacity will drop off rapidly once outside the footprint of the crane $\endgroup$ – Separatrix Mar 27 at 16:50
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Helicopters

Assuming money is no object--this will be expensive, and it will require precision high speed work by skilled personnel. It should result in no harm to equipment or passengers. There are a number of models of heavy-lift helicopters out there; wikipedia has a list. Attach lifting cables, decouple the train cars, and lift them out of the way individually.

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  • $\begingroup$ @Aethenosity the weight objection is valid, but it would be feasible to posit a very abnormally light passenger train, or just hand wave the weight away. (This entire scenario stretches the limits of reason to begin with; most of these suggestions stretch even Hollywood physics.) It's also technically possible to use two helicopters to lift a large load, even though it's borderline insane: aviation.stackexchange.com/questions/24978/… $\endgroup$ – Adam Miller Mar 27 at 20:47
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    $\begingroup$ @Aethenosity, you'd only need five Mi-26s (lift capacity: 20 metric tons) to handle most freight cars. Still outside hand-wave range. $\endgroup$ – Mark Mar 27 at 21:21
  • $\begingroup$ If you have the lifting capacity for the train, just fly the people/stuff you want to transport $\endgroup$ – Baldrickk Mar 29 at 10:21
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former train stops and reverses. Fast train catches up. Passengers swap over to opposite train. Fast train decouples, stops, reverses direction. the former train is now the fast train, and the passengers never slowed down.

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    $\begingroup$ While a sensible suggestion, this answer only works if the trains are both passenger trains and have the same speed and power. Were either of them goods trains, or the slower train not capable of reaching the required speeds, the solution would be insufficient. $\endgroup$ – A Lambent Eye Mar 27 at 15:37
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Magnetic Levitation

In a scenario where a fast train needs to cross a slower train on same track, it might make sense to levitate one of the trains above other.

Perhaps a levitation technology could be built into maglev trains, that allows one train to rise enough to let other pass under it on original track, without requiring other to slow down, and without pulling flying train down when other one passes under it.

Perhaps lower train could have maglev tracks on roof to support flying train when they are passing each other.

I suppose only two problems are facing us, looking at the current technology:

  1. How to balance flying train: train may have tendency to fall to one or other side of track if levitated beyond a certain height. Will need support to balance above track.

  2. Enormous power requirement: Through for a short duration, a lot more power than usual would be required to lift a train a few meters instead of a few mm.

PS: I don't know much about trains.

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protected by James Mar 27 at 19:51

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