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As a way to travel fast, my Earth-like planet is going to have a special train, circling it (the axis of the 'tracks' is aligned with the planet's axis of rotation).

The thing about this train - it never stops and it doesn't end - there is no first or last car, they are all joined in a circle.

Moreover, it's actually several trains, going parallel to each other. The first one travels at the speed of 5 km/h, such that any healthy person can get on it. Then the second train travels at 10 km/h, and they are close enough so it's no trouble to step from one to another. The next travels at 15 km/h, the next at 20 km/h, etc.

I think about 200 trains should be enough to provide the means of fast travel around the planet. Of course, there is the matter of travelling to and from the tracks. As for the 'other side of tracks, we just need a second set of slower and slower trains.

The problems with this concept:

  • Safety of the passengers. If all the trains function perfectly, then we just need a system of safeguards in place so no one could possibly fall down or injure themselves while stepping between the trains. At the speed difference of 5 km/h even if there are some inner walls or support beams, it still should be safe enough. Now on the other hand, if there is some malfunction, the inertia could kill everyone in the fast moving trains.

  • Long time functioning of the train. To get rid of friction we could use magnetic tracks (or even tunnel), but it will take a lot of energy. On the other hand, is it possible to use centrifugal force from the fast trains to support the whole structure? How fast should they move in this case (we can make the inner cars heavier than the outer ones)?

  • What other problems could arise? Interaction with the planet's magnetic field, air resistance currents, etc?

  • Possible means of building the train: the materials (maybe carbon fiber, like space elevators?), the power, the infrastructure?

(!) Important! The only thing I'm handwaiving is the tracks construction. Let's say we somehow managed to do it, despite the mountains, bodies of water, etc. Maybe under the surface, but I won't elaborate further.


I hope the question is not too broad, I will accept partial answer as well. Probably the only question: how to make this means of travel feasible?

I want to make this believable enough so the real fun about the societal implications could start. Can you imagine working as a conductor on this train?

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    $\begingroup$ Uhm. So, if I got your concept right, then I will need to do the following to travel from New York to Berlin: Somehow make my way 4,500 kilometers south to the equator, hop onto the equator train from America to Africa, and then make my way another 5,500 kilometers north again to reach berlin (crossing the Mediterranean Sea on my way). How in the world does that make my travels easier, when the distance NY-Berlin is only about 6,300 km? $\endgroup$ – subrunner Mar 14 '17 at 10:01
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    $\begingroup$ @subrunner - the question specifies "Earth-like planet" not Earth itself - perhaps the major centres of population are all close to the equator. $\endgroup$ – colmde Mar 16 '17 at 8:54
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    $\begingroup$ The concept reminds me of the conveyors in The Caves of Steel. Those aren't going around a whole planet, though. $\endgroup$ – Michael Schumacher Mar 16 '17 at 9:33
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    $\begingroup$ My initial reaction was Terminator City from Kim Stanley Robinson's "2312", a city on rails constantly moving around Mercury's equator, forced along by expansion and contraction from the sun. Check it out. Anyway, getting back to your idea, the biggest thing that struck me was that you'd end up with classes - not just traditional 1st, 2nd, 3rd on trains, but also 'fast' people and 'slow' people, who see life differently according to how often they leave, and what the world looks like as they zoom past. Throwing overboard could be a stern punishment for offenders in the 'faster' classes. $\endgroup$ – flith Mar 16 '17 at 12:34
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    $\begingroup$ @SamWeaver Heinlein's story was called The Roads Must Roll, featuring moving sidewalks (i.e. parallel conveyor belts), where you step from one to the next. $\endgroup$ – ChrisW Mar 17 '17 at 10:39

13 Answers 13

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While hopping on one moving train seems reasonable for a healthy young adult, hopping two hundred seems a little excessive and dangerous...

Also I would imagine that most people going on a long distance trip will be bringing luggage. I'm sure I could jog up and on to a slow moving train unhindered, but a little less sure about doing so with a suitcase in each hand. And I wouldn't even consider it for a family holiday with the crumb snatchers.

Beyond that you need to consider the fuel/energy costs involved in keeping 200 trains in constant motion, not to mention that each train completely encircles the planet. That's an awfully huge investment just for moving passengers. You couldn't​ realistically move freight from one train to the next.

On top of the fuel costs you would have some pretty huge maintenance issues... The trains would cover the entire track, so making repairs to any one section of track means shutting the entire system down and removing several train cars before repairs could even begin. Likewise repairing and maintaining​ the individual trains would mean shutting down at least most of the system.

Then there's the worst case scenario...

Should any one of the trains derail you're looking at a really, really, really ugly domino effect. Your trains would have to travel really close to one another to allow passengers to hop between them... Imagine someone drops their luggage on the track making the hop. A hard metal briefcase slips under the wheel... The car tips just enough to bump against the next faster train, which in turn bumps the next, and so on... This tiny ripple could and likely would build into a tsunami of grinding twisting metal.

See: https://en.m.wikipedia.org/wiki/Derailment


I sort of like the idea of an infinite train circling the planet that never stops though, so I thought up an alternative system. Rather than having 200 circumnavigating trains you could try 1, with several "feeder" trains.

Basically you have your mainline (red), traveling at a reasonably fast and constant speed. Then you have secondary or feeder trains (blue) like so:

enter image description here

Your red line never stops, but your blue lines more or less behave like normal trains. The blue lines make regular stops at stations, load passengers and freight, then speed up to match the speed of the red line for several miles allowing for a "hard dock" procedure to take place between the two moving trains where passengers and freight are passed between the two trains.

This sort of system could minimize some of the aforementioned safety concerns and would require substantially less energy.

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    $\begingroup$ This feeder/main train also resolves some of the strain for handicapped riders who literally can't "hop." And given that there are fewer trains in your circle, it would consume less real-estate width-wise. $\endgroup$ – CaM Mar 13 '17 at 20:46
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    $\begingroup$ The problem with the feeder train setup is that if you can get the feeder train up to speed, why not just have it transfer onto the long track and then disembark when it gets to its destination? That alleviates the need for the constantly moving train and the transfer mechanism (which is probably the riskiest part of the system). But once you've taken that part out, you are left with a normal train system that works like the ones in real life (there is a reason we use the system we use) $\endgroup$ – Kevin Wells Mar 13 '17 at 21:28
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    $\begingroup$ @KevinWells: The feeder train could be much smaller and lighter (and thus easier to accelerate and deccelerate). I remember seeing a design like that on a Discovery, though I think it was about an underground vaccum train, which would go accross Europe (and possibly under the ocean, to North America), or something of the sort. The feeder train would attach to the top of the "main" train. $\endgroup$ – tomasz Mar 13 '17 at 22:17
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    $\begingroup$ @KevinWells: Why not just have the feeder train transfer onto the long track and then disembark when it gets to its destination?  What destination?  The point of a globe-encircling system is economy of scale.  If person A wants to go from station 101 to station 142, and person B wants to go from station 102 to station 143, they both want to be able to do so with two transfers and not 40 intermediate stops.   (Granted, I believe that the OP’s scheme has big problems, but that’s why @apaul34208’s suggestion is what it is.) $\endgroup$ – Peregrine Rook Mar 14 '17 at 0:55
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    $\begingroup$ @Random832: Clever. Make it a SAS (that is, both doors are NEVER opened at the same time) and it solves the issue of both doors being locked... or actually, make it just work that way. Think of it as "an exchange capsule". People wishing to disembark step into the capsule. Upon arriving to the feeding zone the train offloads the full capsule of people wishing to disembark and onloads the full capsule of people wishing to embark. This way, you always have plenty of time to get in/out the capsule, and the exchange at the "feed zone" can be real quick and not worry about elderly, suitcases, etc.. $\endgroup$ – Matthieu M. Mar 16 '17 at 14:23
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A similar concept has been mentioned in other answers, but one way to make a train like this viable would be to have pods at stations that "dock" with the main train rather than many trains running at different speeds. There would be a period of acceleration followed by the pod locking on firmly.

Here's an example where a pod docks to the top of a train at each station.

gif version

Passengers would then move down from the pod to board the train. The pod then unlocks and decelerates to zero just as it comes to the next station.

While the pod in the animation is shown on rails, this could easily be another type of vehicle. One example could be an autonomous drone that performs roughly the same motion as the pod on rails, but hovering slightly above the train without a second set of rails involved. This would simplify the system physically, and take advantage of modern advances in computing.

The core concept here is that you want to accelerate a small mass to meet up with the large mass, constant velocity train for reasons of energy efficiency.

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    $\begingroup$ Using the third dimension really simplifies the previously given solution rathre a lot! $\endgroup$ – Jasper Mar 14 '17 at 10:05
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    $\begingroup$ Methinks the cute animation wildly exaggerates the plausibility of this. Small omitted details like the fact that trains aren't actually composed of a single, straight, connected, section would actually pose huge problems. $\endgroup$ – Jack Aidley Mar 14 '17 at 11:34
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    $\begingroup$ @JackAidley we're already using hypothetical technology in this answer, the issue with a single body would probably be pretty easy to solve. $\endgroup$ – DonyorM Mar 14 '17 at 13:14
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    $\begingroup$ @DonyorM: It's just one of what would actually be a vast host of problems with this idea. It's a cute animation but don't be fooled into think it presents anything that is actually plausible. $\endgroup$ – Jack Aidley Mar 14 '17 at 13:18
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    $\begingroup$ @JackAidley Thanks for the feedback. I've updated my answer to include the possibility of using autonomous drones for the pod to avoid the issue of a second set of rails and a long section of train that is perfectly straight. $\endgroup$ – jncraton Mar 14 '17 at 13:59
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Track maintenance: This one is actually easy to address. Everyone is stuck on what a train is: two wheels on two rails. Lets make a different train: Four wheels on four rails.

Now the maintenance crews can come along underneath and remove (carefully!) any section of rail and replace it. Likewise, a crew on the train can open a panel and lift out any wheel. With sufficiently modular construction every moving piece can be replaced while the train is in motion.

This reduces the problem to dealing with corrosion and metal fatigue in the frame of a train part. To solve this you have some special train cars that takes an entire car in, lifts it off the track for the heavy maintenance or even total replacement, and then puts it down and advances to the next car.

Safety when crossing to the next car: Lets put some more wheels on the train--this time on the side. In addition to the ground each car rides "on" it's neighbors. This is purely to keep a very precise spacing compared to that neighbor--the gap between the cars is a small fraction of an inch. Note that you will have to have thermal expansion capability elsewhere in the car or the whole thing will become a spectacular disaster if it warms up.

5 kph is a small enough difference that a healthy adult would have no problem crossing the divide. It should be no harder to navigate than current-day slidewalks are. However, this would be a big problem for the young, the elderly and the infirm or disabled. You would be forcing an awful lot of people into power wheelchairs that would otherwise be quite capable of moving on their own.

Also, it would take an incredibly peaceful world for something like this to be tolerable. Imagine how destructive a bomb in the middle of things would be.

I also have a hard time imagining the traffic volume that would make a system of this nature make sense. For the cost of a system like this you could build a huge number of stations along the lines of what (jrcraton) proposed, except with multiple tracks moving at different speeds. (The faster tracks would have stations farther apart. (Your 1000 km/hr track has a minimum distance of 10km between stations assuming a system with different capsules being boosted and slowed. I'm figuring a half gee on the boost.)

If you don't like his system of the capsule riding above simply have separate powered train cars. The car boosts on a parallel track, when it's going fast enough it switches over and connects to the head of the train. The tail car is dropped and decelerates into the next station. (Two tracks mean 20 km separation for a 1000 km/hr train.) This does require passengers to move about on train although they need not move all that fast--every car would have a big electronic board saying where it was going, if your car is in the train you sit down there. If it's not you have to keep walking forward at the rate of one train car every minute. (Which suggests there would be some slow trains--the station spacing would be longer than normal, lower boost and a lower walking speed needed to stay ahead of the shedding tail.)

I've come up with a better way to build the trains:

Build two types of train car. One car is for getting on and off, one is for travel. You get on a train car, it's boosted to the speed of the target train and gets in front of it where it couples to the train. The main travel tracks have 4 rails, not two, the boarding car rides on the inner two rails. The outer two rails hold another car that is always moving at speed and surrounds the boarding car. The boarding cars move slightly slower than the permanent ones so they slowly slip to the rear, the speed would be something like one car length per minute. Once the boarding car is within the main car you can step over to it. The main cars hold things like restrooms and perhaps some shops. Each boarding car has a sign indicating where it will go when it detaches, when your objective shows up you go sit in that car.

The main trains would come in various speeds, the track layout alternates between the two rail tracks for the boarding cars and the four rail tracks of the main lines. The highest speed train probably would only be accessible by boarding cars switching from a lower speed train.

Conventional light rail reaches a capacity limit based on how long a train has to sit in a station for loading and unloading. This system, however, can support a huge number of trains on the track, although any given station will still have a capacity limit (albeit much higher than for a conventional design) and the need to provide frequent stations doesn't set a speed limit on the overall transit.

I think four trains would be about the practical limit: 20 km/hr, 80 km/hr, 320 km/hr and whatever the engineers can do without causing mach problems.

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    $\begingroup$ Modern travelators have two important differences to this, firstly they move more slowly (below walking speed, around 3kph; excepting a few high speed travelators that have special acceleration/deceleration zones) and secondly the person meets in line with the direction of movement. Leaping sideways onto a platform moving at 5kph is altogether more difficult and it is likely that even healthy adults would fail to do so with reasonable frequency; the large proportion of the population that don't fall under "healthy adult" would have altogether more trouble. $\endgroup$ – Jack Aidley Mar 14 '17 at 10:00
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With 200 trains and a 5km/hr speed difference between them, the fastest train is only going at 1,000 km/hr. That's barely faster than a passenger jet, which goes at about 870 km/hr, which doesn't seem to be worth the considerable effort and inconvenience.

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You would need a lock system where persons traveling from train to train would step into a pod.

Once the doors closed behind you, the train your passengers are on would send a signal to a pod on the destination train to have a matching pod extend to the current train, once the two pods aligned, the source pod would be pulled from the source train closer to the destination train. When the passengers departed from the pod to the destination train, the doors would close behind them and the pods would extend back out, and deposit the pod back to the source train.

Each train, on it's source side could have tracks and gaps to allow for repositioning of the pods.

This system would provide complete safety while transferring from train to train as even someone in a wheelchair could do it.

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No matter what kind of system you use, maintenance is necessary. Maintenance can be broken into two categories, maintenance of the cars and the tracks. Apaul's idea about having a separate speed matching train just for boarding makes maintenance, much, much easier. The system of hundreds of parallel trains means that the inside trains and tracks are completely inaccessible.

If you have a feeder train for boarding passengers, why don't the feeder trains swap out entire cars? One or more train cars exit the track and are simultaneously replaced by new cars, with new passengers and new cargo. This allows the cars to be removed for maintenance.

This presents some engineering challenges, such as catastrophic derailment, but you have figured out a global train system. I assume your engineers are up to the challenge.

To make the engineering challenges easier, make the train capable of having cars missing. In a conventional train, removing one car while the train is moving disconnects the whole train. This train has a linkage on top that stays when a train car is removed. This linkage bridges the gap when a train car is removed, and transmits the tension force that the missing car would have been carrying. The train should be able to function with a decent fraction of its cars missing.

In order to maintain the tracks, have two sets of parallel tracks and the train can be switched from one set to the other set.

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  • $\begingroup$ Regarding maintenance... what if the train is composed of "long enough" cars that they can accommodate a gap in the rail? So that you can, from below, remove a portion of rails and replace it with the train going full speed on top? Would make for crazy imagery. $\endgroup$ – Matthieu M. Mar 14 '17 at 12:20
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Lets assume we somehow manager to make something like this. The fastest train would be travelling at 1000Km/h (200*5). To get down from the train one has to cross all the 200 trains again. In addition to that if there is a delay in switching trains (especially if he is on one of the fastest trains) a person will miss his station and would have gone far away by the time he reaches the slowest station in order to get down from the train.

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I started this post assuming the idea of building that many train cars was ridiculous. It turns out that it may crash the world economy but the material is actually there.


I'm posting this as an answer instead of a comment simply due to the length. My numbers all come from the internet so there's a grain of salt in all this. I am not a train engineer; I am a toast engineer. Your trains may easily be more advanced that the current Amfleet so they can be lighter, but let's see what happens if you build a single, circumferential train with today's technology.

DATA
Train Car Weight: $49,500$ kg avg. (source)
Train Car Length: $26.01$ m (source)
Train Car Max Speed: $201$ km/h (source)
Earth's Equatorial Circumference: $40,075.017$ km (source)
Earth's Crust's Mass: $2.6\times10^{22}$ kg (source)
Delta in Train Speeds: $5$ km/h (source)
% of Steel that is Iron: $90\%$ min (source)
Annual production of iron: $1.1\times10^{12}$ kg (source) (2012 data)


CALCULATIONS (Numbers shown are rounded for simplicity but calculations used the precise values given above.)

Cars required to encircle the equator once: $\frac{4\times10^7m}{26m}=1,540,754$

Total mass of those cars:$1,540,754\times49,500kg=76,267,323,000kg$

Number of tracks required to reach max speed: $\frac{200 km/h}{5 km/h \mathrm{\ steps}}=40\ \mathrm{tracks}, 79\mathrm{\ for\ mirrored}$

Total mass of all cars on all tracks: $(7.6\times10^{10}kg)\times79=6\times10^{12}kg$

Mass of iron in the Earth's cust: $(2.6\times10^{22}kg)\times5.2\%=1.356\times10^{21}kg$

OK, so we have 225 million times more iron than we need. That's good. Considering that some of the car's mass is other material like fabric, electronics, and wood, we actually have an even higher safety margin. How long is it going to take to get that much iron, though?

At the 2012 rate, it would take only $\frac{6\times10^{12}kg}{1.1\times10^{12}\frac{kg}{yr}}=5.5\ \mathrm{years}$ of consuming all iron production in the world to have enough for your train system. That's surprisingly reasonable although it would have a drastic impact on the economy. You could reasonably spread the project over decades since it will take so long to actually build the thing and that would lessen the impact.

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  • $\begingroup$ Steel is not the best material for such a project. I was thinking carbon fiber $\endgroup$ – Yuriy S Mar 15 '17 at 15:07
  • $\begingroup$ That would take less energy to power but it would cost a lost more. It would also take longer. The 2016 production of carbon fiber was $1.3\times10^8$ kg. That means we mined over 8,000 times more iron in 2012 than carbon fiber produced in 2016. That production would need a huge boost before it could build the trains. $\endgroup$ – Engineer Toast Mar 15 '17 at 15:20
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One problem is getting scissored in the doors between trains moving at different speeds.

You might want to consider getting rid of those doors completely by using the moving roadways from the Heinlein story Jeff Zeitlin mentioned but have covered sections to protect against weather/wind. An alternative would be to have a covered center section and a open "hallway" on either side. To be even better, have sections of the hallway (or the whole thing, if you are being generous) move to bring it to the halfway point of the speed difference.

One issue that i see here that I don't recall Heinlein dealing with would be the weather generated by the trains. The trains would create prevailing winds around the globe that might spin hurricanes off either side (look at the closeup of Jupiter's bands).

Also, unless you can get over or under this train, it becomes a barrier to travel from north to south.

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    $\begingroup$ As I recall, Heinlein's roads (and Asimov's) were enclosed. $\endgroup$ – Mark Mar 13 '17 at 22:09
  • $\begingroup$ It's been a while since I read those. Even so, can you imagine the wind in whatever tunnel they are running the trains in? $\endgroup$ – ShadoCat Mar 14 '17 at 0:41
  • $\begingroup$ Building bridges over and/or tunnels under the train track is a simple matter. $\endgroup$ – Peregrine Rook Mar 14 '17 at 0:53
  • $\begingroup$ Yes but it would still be a barrier. You see it now with railroad tracks that you can drive over. If you have ever heard the term "the bad side of the tracks," even a purely psychological barrier can have a big impact. $\endgroup$ – ShadoCat Mar 14 '17 at 1:05
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    $\begingroup$ I have read Heinlein's version of the system more recently than Asimov's, but my memory is that certain tracks had partitions running in line with them so that air currents could be matched to the average speed of the tracks for that section. A constant 10kph wind for the 0-20kph section, a 30kph wind for the 20-40 section, etc. $\endgroup$ – Rozwel Mar 14 '17 at 17:25
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It maybe would be safer if every second train shifts between the velocity of the two trains on each side of it. Train 1: Stops in 1 minute, 10 km/h in 1 minute. Train Train 2: Always 10 km/h. Train 3: 10 km/h in one minute. Then 20 km/h in one minute. Train 4: Always 20 km/h. ...and so on. During the minute the trains has the same speed it is easy to walk between them. Then the doors closes when it's time to change speed.

With this variant you could have much higher difference between the trains, maybe only having two trains: one that never stops and going at very high speed, and one that accelerate up and down. Then every passenger only needs to accelerate one time and decelerate one time, during there travel.

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  • $\begingroup$ Acceleration up and down will greatly increase the wear and tear on your trains and cause them to require more frequent maintenance. $\endgroup$ – Ross Presser Jun 18 '18 at 17:40
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What is the distance between stations?

Whatever it is, the passenger (possibly elderly or disabled) needs to be able to mount on the train at station A and dismount at station B in less time than it takes for the train to move between A and B. Otherwise he won't be able to get to the intended station.

To achieve low transit times we need a speed of at least 1,200 km/h, and this requires shielded tracks, possibly underground. Also, this way we stay subsonic.

The user enters a stationary car and the car starts accelerating at 1 m/s, which is quite typical for metro trains - you don't want to do more than that, I think - and in six minutes you're going at full speed. In those six minutes you've covered about sixty kilometers.

Supposing the stations are twenty kilometers apart, you need at least three "acceleration lanes" for cars to dock with the train (better make that five or even six). You will also need as many deceleration lanes for cars to undock from the train and reach the stations.

Is this enough? No.

Once the user is on the train, he needs to reach the undocking car. How long does this take? It depends on how many cars are there on the train. Logically, the car will have docked near the cars undocking next, so that if you want to do a short hop, you just need to stay in the same car:

Station  Detach Accelerating Docking Detach   Decel  At station
Abel     A
Baker    B      A
Court    C      B A
Dewey    D      C B         A
Earl     E dock D C         B
Fawn     F      E D         C        A
Green    G      F E         D        B        A
Hall     H      G F         E        C        B A
Ivy      I      H G         F        D        C B    A
Jeans    J      I H         G        E        D C    B
Kell     K      J I         H        F        E D    C

So you mount at Abel, dock with the train while the train is passing Dewey, let people disembark that are not directed to Ivy while Earl goes by, detach from the train at Green, dock again at Ivy. The next car departing from Ivy will be Abel's car.

If you mount at Abel and want to go to Kell, you need to exit the A car, you pass by the B car that's now docking, you reach the C car that's flashing "INBOUND FROM COURT - OUTBOUND TO KELL", and sit down. At 1000 km/h, the train passes each station every 1.2 minutes, so this (plus the disembark time - another 1.2 minutes; but we can play with that) is the time it must take you to move from one car to the next.

Also, you can take about six minutes to sort yourself out to leave the car before it docks.

For "metro" style of travel, provided it's not rush hour, this is probably enough. Otherwise we need to provide a longer dock time; but every 1.2 minutes we give it's another station that flashes by.

So what if you are at Abel and want to go to Hall, instead of going to Kell? You can't. The minimum is eight stops, and Hall is seven. What you need to do is to take the opposite train and travel to Nook, eight stops sunwise, easy travel - sit down, get up. From there you disembark and take the widdershins train to Hall, fifteen stops. You need to "walk" 15-8 = seven carriages, exactly the number of stops between Abel and Hall.

Basically, when you arrive in the train, you find yourself in a hall/corridor with doors marked "Hall", "Ivy", "Jeans", "Kell" and so on. And the section of corridor you're in starts to darken :-) - after four cycles it will light up again but it will now say "Storn".

A travel will involve either going forward for a maximum number of stations given by acceleration time + deceleration time + docked time, or sit in the hall/corridor the required number of "cycles".

All things considered, it might be interesting to design the train as a luxury shopping mall with sleeping accommodations. You can wander the "travel area" and in about 22 hours you can be on the other side of the planet.

Basically you would have two very large tunnels (or tubes, if outside), one for each direction. Each tunnel is divided in one "fast" part with one carrier lane, and one "accelerating/decelerating" part with several lanes. The cars leave the station and accelerate; if everything is OK they leave the accelerating lane after matching speeds with the train, and dock with it. Otherwise they proceed in the decelerating lane towards the next station. The "fast" lane would have a series of carrier trains running one after the other. To get one down for maintenance, you stop accepting carriages to it (all departures are delayed by one inter-train period and cars attach to the carrier train coming immediately after), send off all carriages until it is empty, then detach it from the line. Inter-train period must be at the very least the time required for the carrier train to emergency brake and the whole world line to come to a stop.

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You need to make the tracks immune to flooding, earthquakes & other accidents & 'acts of god.' With one contiguous rail system using a single route.. a single such localized accident would shut down the entire world economy.

The single 'circular' line would be more practical on a space station or suchlike where the environmental conditions are entirely managed.

The pod idea referenced in other answers would need to have the pos accelerating to catch up not simple because transfer is otherwise implausible, but because if you suddenly accelerate a person from 0 relative motion to 1000km/h (or vice versa) they die, it's as simple as that.

So far as energy efficiency and such-like are concerned, you're still having to accelerate each pod/car on and of to the central rail.. and whether you achieve that sequentially or in a single hop the total required acceleration is the same.

We still have the problem of local transport (unless the entire populated area is along the equator, which is feasible of course, the climate of the planet might require it, or etc.) But on a planet like earth we have non-contiguous land masses that aren't served at all by this method.

Just problems, yes, but hopefully they'll help you refine your final answer.

Perhaps the train line is in a hermetic system tough enough to withstand any conceivable trauma, underground and self-supporting to such a degree that earthquakes that shift entire geographic regions up and down by a meter or so don't disrupt the system. The tunnel is a vacuum like elon musk's proposal so as to reduce energy costs for acceleration & deceleration of cars.

Vacuum also reduces corrosion effects, so you're only having to repair under normal conditions for mechanical erosion as opposed to rusts and such.

Of course though, if you have a breach of any size on a world-spanning vacuum tube..well.. the consequences would be pretty massive.

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Use conveyor belts rather than traditional trains

Depending on what your definition of trains is, you may consider another option which is IMO safer.

Each train is actually nothing but a conveyor belt with seats placed on it. There are no walls or a roof. So you have a large number of conveyor belts running at different speeds, each with a width of some 5-10 metres. The seating can be a lot more spacious and your species could even live on the belts if they aren't prone to road sickness :P

It is easier to transfer yourself and your luggage across parallel conveyor belts rather than trains which have walls for obstructions. Handholds can be provided to make it easier to maintain one's balance.

An even safer (but costly option) would be to have automated wheel chair seats at the borders of each belt. You just sit on the seat, push a button, and it manoeuvres itself onto the next belt by clinging to supports. You get off the chair and sit on a normal chair. If you don't want to roll your luggage across, you can place it in another box that also makes an automated manoeuvre to the next belt.

P.S. I was just thinking, sitting on such a belt would be fun; watching people nearby moving at different speeds.

P.S.2 If having continuous electricity to run the belts is expensive, one can divide each belt into chunks, each chunk being a few hundred metres or a few kilometres long and pulled by a single engine. The engines act as links within each belt, like a kind of chain. It might be easier to adjust derailed belts or replace old belts in such a setup (as opposed to a single infinitely long belt).

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