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In my world (essentially playing in the late 1920's to early 30's, though with a bit of hand-waved technological and political progress), a rail line exists that spans Eurasia from Lisbon to Bangkok.

Most segments of the tracks are either completely straight or only bent in a very minimal way, so there wouldn't even be a need for the train to be articulated to a great extent.

I'd imagine the trains that run on these tracks will probably have to travel in excess of 200 km/h to get anywhere on a reasonable time scale.

The Mallard is, to this date, the fastest steam locomotive ever built, having gone about 203 km/h (126 mph), though I've been wondering if it's possible for a steam locomotive to go even faster, or if there's a physical/structural limit to their speed-

Steam turbines are an option as well (like on the PRR 6200), but I'd really love to include connecting rods for their design, if possible. Diesel engines aren't part of the question though.

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    $\begingroup$ I mean for all practical intend the speed of sound is probably the limit. $\endgroup$
    – ErikHall
    Aug 5, 2023 at 13:08
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    $\begingroup$ Do you have a specific goal in mind? We can replace all the metal with a modern, high-strength, lightweight alloy. We can use modern techniques to increase steam pressure. We can design it for low wind resistance. We can design the wheels to hold onto the rails to minimize tipping over. We can design it to have a lower center of gravity. We can design the tracks to be as smooth as possible and have little to no curvature... And after doing all of that, it might get to 200mph. But what's the point of the question? What problem are you trying to solve? (A Stack Exchange requirement.) $\endgroup$
    – JBH
    Aug 5, 2023 at 14:38
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    $\begingroup$ @JBH "if it's possible for a steam locomotive to go even faster, and if there's a physical/structural limit to their speed" because "Id imagine the trains that run on these tracks will probably have to travel in excess of 200 km/h to get anywhere on a reasonable time-scale". That's the problem! The fastest locomotive ever built in our world went up to 200 km/h and the locomotives in my setting will probably have to exceed that speed, and i want to know if that's physically possible for that type of train. $\endgroup$
    – NimRad
    Aug 5, 2023 at 14:54
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    $\begingroup$ Another factor to consider is the grade (slope). A train of a given power will not be able to maintain the same speed going uphill as on level track. And it has to limit speed going downhill for safety. Plus a train (exception: light rail subways, etc.) can take a long time to get up to speed, so you want to minimize the number of stops. $\endgroup$ Aug 6, 2023 at 3:51
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    $\begingroup$ "to get anywhere on a reasonable time scale" is something that appears to be coming from your early 21st century point of view. What was considered "a reasonable time scale" for travel has varied dramatically over time and is completely dependent on the preconceptions of those who are traveling/shipping as to what is "reasonable". We currently consider hours to be reasonable for trips which used to take a year. For a train, you'd have multiple trains traveling at the same time along the track, so the aggregate volume moved is higher, even if the entire trip took many days. $\endgroup$
    – Makyen
    Aug 6, 2023 at 20:45

8 Answers 8

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For steam locomotives, the limits of speed are affected by mass, friction, aerodynamic drag and power. As stated in the question, the Mallard was the fastest steam locomotive, and it achieved this by virtue of its streamlining.

The PRR 6200, as a steam turbine driven locomotive was not much slower than the Mallard, 110mph/177kph vs 126mph/203kph, and it was not streamlined. Apparently, it suffered from the disadvantage that its steam turbine was directly coupled to its wheels.

Extrapolating from those two locomotives, a streamlined steam turbine locomotive with a 2 or 3 speed gearbox, or an electric drivetrain could conceivably exceed the Mallard's speed record, though I would not expect speeds to exceed 150mph/241kph under light, streamlined load conditions.

With weight-saving measures such as titanium construction and an emphasis upon lightness for the entire train, speeds of up to 175mph/281kph may be achievable.

The problem with steam engines is that they require large quantities of both fuel and water. By making the steam turbine a closed-loop system with a condenser, the necessary mass of water can be reduced considerably, and by using fuels such as heavy fuel oils, the mass of fuel can be reduced, as well as eliminating the need for both a fireman and a driver.

Reciprocating pistons have the problem that they suffer from vibration at speeds other than that for which they are designed, which causes mechanical stress and limits top speeds.

However, even with a geared or electric, closed-loop steam turbine, streamlining and titanium construction, I would expect a locomotive on a regular passenger service might typically reach speeds on the order of 150mph/241kph.

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  • $\begingroup$ Yeah, even a steam-turbine locomotive with high-spead gearing probably won't do any better than the UAC TurboTrain or another gas-turbine set did, which is not nearly as good as what you can get by having an external power source instead. $\endgroup$
    – Shalvenay
    Aug 6, 2023 at 4:59
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    $\begingroup$ I'm not sure how much of an issue weight is for top speed. I suppose it lets you use longer gearing which may help. The Mallard is the most aerodynamic steam locomotive ever made, which is not the same as being the most streamlined which could be made. Extra aerodynamics (especially to stop air getting under the engine) combined with a 2 speed gearbox would probably be enough to get to 250kph. $\endgroup$
    – Turksarama
    Aug 6, 2023 at 6:28
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    $\begingroup$ Gearboxes on locomotives are avoided for a reason. They'd be impractically large for anything other than a single speed. Given the limits of single speed gearing there would be a compromise on the power produced at slow speeds and the maximum speed the wheels could turn under any power. The best, perhaps only, way we've found to get around this is an electric drive. There's no closed steam loops on locomotives for the same reason, a condenser would be impractically large. $\endgroup$
    – MacGuffin
    Aug 6, 2023 at 6:46
  • $\begingroup$ @MacGuffin An electric drivetrain would probably be better, and more efficient, but there are compact multispeed gearboxes that might work at locomotive scale. The entire train's roof area could be used as an air-cooled condenser surface if necessary. $\endgroup$
    – Monty Wild
    Aug 6, 2023 at 7:09
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    $\begingroup$ turbo-electric steam locomotives have been tried & their problem was being very complicated, expensive, unreliable, & heavy. Every attempt at turbo-electric steam locomotives has had this happen. Union pacific managed to fix unreliable with theirs but the rest of the problems they never could. $\endgroup$
    – OT-64 SKOT
    Aug 7, 2023 at 3:28
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You have a few assumptions here which are very much based around normalising current infrastructure. There are a few things you might need to reassess.

and if there's a physical/structural limit to their speed

The limit on train speeds has much less to do with the train than you'd think. The main limiting factor for trains, since before Mallard, has been track quality. In the UK, Intercity 125 trains never ran at 125 mph in normal service - average speeds were always under 100 mph, a speed which the existing Deltics could already achieve, because the track wasn't smooth enough for them to run any faster.

Wider gauge tracks would certainly improve that, of course. There's a good reason that Brunel set a 7ft gauge on the Great Western Railway, and many current high-speed routes use wider gauges.

so there wouldn't even be a need for the train to be articulated to a great extent

Again, track is your issue here. You're grossly overestimating what "straight" means. Sure, your trains wouldn't need fancy tilting rolling stock to take corners fast. But you'd still need multiple carriages with articulation between them, because there's a limit to how straight and level you can fit rails, and how straight and level they will stay over time. You could maybe make them a bit longer, but not significantly more. Regular railway carriages in the UK are 60ft, whereas the Shinkansen carriages clock in at 82ft. Bigger, but not dramatically so.

Id imagine the trains that run on these tracks will probably have to travel in excess of 200 km/h to get anywhere on a reasonable time-scale.

Now this is your big assumption. Before jet passenger flight became common, "a reasonable timescale" was simply as long as it took, because there wasn't an alternative. Sleeper trains were the norm for any kind of long distance trip, and you just planned for however many days it took to get there. Russia still runs trains through Siberia, and Australia still runs trains across the Nullarbor Plain. The fact that it takes a few days to get across a continent is not exactly a limit.

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  • $\begingroup$ The track does run through Siberia for the majority of its length, and it actually is a broad-guage by design, but you're right. I mainly used the "reasonable time scale" terminology to narrow down the question a bit, because a track of this length would still take days no matter what. I guess the environment and materials might be the limiting factor then, because of how the tracks could get warped by withering and temperature. $\endgroup$
    – NimRad
    Aug 6, 2023 at 0:14
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Your problem would be operating economics

The thing with a steam locomotive is that "an hour on the track is an hour in the shop", even at conventional speeds. Given that Mallard didn't survive its record run completely unscathed, the rigors of such high-speed running would take a toll on just about any steam locomotive design to the point where the economics of maintenance would favor just about anything else for regular high-speed revenue service.

As a result, you'd probably see mainline electrification on such a line as soon as someone ran the numbers on how much they were spending to maintain their locomotives, as well as the increased track maintenance costs that'd be caused by the quite severe pounding loads associated with a steam locomotive.

Why not steam turbines?

While steam turbine locomotives existed back then -- these units used a direct-drive or single-speed geared system, sometimes with intermediate crank arms in the drive mechanism as well. This made them very steam hungry at low speeds, and the fine mechanics of a steam turbine would not help the topic of maintenance costs either.

Electrification was already viable then

This leads us to the solution that was already in play then, and what modern HSR above the 125mph/200kph mark almost universally uses as well: overhead electrification with AC power. The 11kVAC 25Hz New Haven electrification had already been in service for two decades at that time, and the performance benefits of not having to drag a generating plant around with your train were well-known in the art even then, with the Pennsylvania Railroad installing a similar 11kVAC 25Hz system in what is now the Northeast Corridor at that time. (That PRR electrification is still in service today with some minor upgrades and plant renewal over the years -- the only reason it's not capable of 150mph+ operation "as is" is due to catenary tensioning issues.)

Electrification has other benefits as well. Electric traction eliminates pounding loads, and also makes multiple unit operation eminently practical, where the traction motors and controlgear are spread throughout the coaches instead of being all in a locomotive. This also plays nicely with the notions of streamlining that became popular in the late 1920s and early 1930s, based on early wind tunnel work and the development of the Jacobs bogie that improved high-speed dynamics greatly.

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Disclaimer:

The answer from Monty Wild covered the mass optimization part, except for the fuel. Others have also answered with stuff about rail quality issues etc. and I have nothing to add there. That being said, there is one more technology path that could be well worth exploring.

Going nuclear

Lets fit a nuclear reactor on your train.

  • It will provide all the heat you could ever want
  • It will get rid of the necessity of having a huge haul of coal attached to your train.
  • Also since you will directly use the steam, you don't need to concern yourself with converting the steam to electricity.
  • As an added bonus you will get rid of having several shifts of coal-feeders(or whatever the term was) and will instead have just few engineers making sure the reaction stays nice and tame.

But what about the radiation? Won't the shielding prove heavier than the coal? In 19th century we didn't care all that much about stuff like that. So just add some lead plating around the engineers outpost and call it a day. Who cares about a teensy bit of radiation somewhere in the deep wilderness?

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  • $\begingroup$ This was my thought as well. It's kind of funny, but modern day nuclear reactors are literally just steam engines. Nuclear reaction heats water and drives a steam turbine. I figure if you can power a city off of a nuclear driven steam turbine, you could presumably make a pretty fast train, too. (Or, if you put a nuclear reactor on a train, and use that to run electric motors, isn't it still, basically, a "steam engine"?) $\endgroup$
    – JamieB
    Aug 8, 2023 at 18:35
  • $\begingroup$ Regarding the radiation issue, there's also the fact that water is an extremely good radiation shield, and while water in effective amounts might eat up a significant chunk of "weight budget" it'd be essentially free on the economic budget. I have no idea how one would justify them knowing about the radiation shielding of water, but on that same note how do they know how to build nuclear reactors? $\endgroup$ Aug 8, 2023 at 19:55
  • $\begingroup$ I was just about to comment this. Nuclear reactors didn't exist in the early 1930s, but pushing them forward 20 years doesn't sound implausible. It may be, I have no idea, but to the untrained ears it doesn't sound implausible. $\endgroup$ Aug 8, 2023 at 21:17
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No way can say, "The top speed for a steam locomotive is X kph." It depends on many factors. If you reduce the total weight of the train while keeping the power of the engine the same, then it will go faster. Add better streamlining.Reduce friction between wheels and rails. (Though frankly, this is a pretty minor factor with trains.) Etc. Can the power of the engine be increased without increasing its size and weight? Probably. I'd be reluctant to say that any technology has reached its absolute maximum. But a lot of work went into steam engines so my guess -- and it's just a guess -- is that without some radical new idea, they're probably near the maximum achievable. If you're writing a fiction story, you don't have to actually provide design specs for your improved train. And indeed most readers would be bored out of their minds if you did, especially if this is not a key element of the plot. How fast do you think your trains need to be? If you said they travel at 200 kph, I doubt any reader would question it. If you say they travel at 1000 kph ... yeah, probably not plausible.

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    $\begingroup$ You really don't want to reduce wheel to rail friction, it's already so low that sanders are required to artificially increase friction to prevent wheel slip. $\endgroup$
    – Monty Wild
    Aug 5, 2023 at 21:33
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    $\begingroup$ For steam engines, the limiting factor isn't power-to-weight ratio, it's balance. Run the pistons too fast, and the locomotive shakes itself to pieces. $\endgroup$
    – Mark
    Aug 6, 2023 at 19:04
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So, there are multiple factors in the history of modern train development which you are mostly missing with a single engine.

  • Breaks. It was established quite early, but breaking on the wagons and not only on the engine might require manual labor – or better hardware.
  • Refuelling and refilling with water – you need many stops for a steam engine.
  • A limit to the power of a single engine: you can make a single engine only that powerful and only having so many weight (to be able to pull many wagons behind it).
  • A corollary: the amount of weight a wagon coupling system can allow.
  • Air drag (as mentioned above, all that streamlining).

The solution to many those problems are either powered wagons (in passenger trains, which are basically all electric) or multiple engines. In both cases you (basically) need to control multiple engines as a single one. This is, however, not viable before some electronic control system.

It might be a quirk of our technical development, but by the time we (as humanity) could reliably control multiple engine units in a train, we had electrical / advanced diesel trains.

Why am I talking about power and number of wagons? Because if you want to accelerate a train to a higher speed, you need pull power and braking ability which you basically could use to pull more wagons at a lower speed. (It does not convert completely, but it's the same direction.)

Next requirements are the economics: Why do you want to pull the trains so fast?

  • Cargo ain't the reason.
  • Passenger trains could just spend a further night without much ado.
  • If it's really urgent, they could just cable the message. Or fly (if flight is in the picture already.)

Also, if you follow the Transsib route for a while, it had some problems historically. It was not easy to build with all the bridges and tunnels. So, it's again the elevation and load limits to the trains.

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    $\begingroup$ The economics are straightforward: in the time period in question, carrying the mail quickly was a strong incentive to move faster. Sending a message by telegraph was expensive, and a single mail car on a passenger train could carry as many messages as the entire telegraph network. Competition between passenger lines also works as an incentive, though it's probably not present here. $\endgroup$
    – Mark
    Aug 7, 2023 at 2:14
  • $\begingroup$ Electromechanical Multiple Unit control was first employed on rapid transit cars in the late 1890s, so it would have been well known at the time of the OP's setting $\endgroup$
    – Shalvenay
    Aug 7, 2023 at 4:22
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    $\begingroup$ Regarding stopping for water: It can be replenished on the fly, using a long water trough, with a scoop on board the train. This tech was in use already before 1900, and not uncommon on some express routes in the UK and otherwise. See en.wikipedia.org/wiki/Water_trough. Regarding multiple engines: You don't need mechanical control systems, just a second set of driver + stoker in each additional engine. Communication could be done with the whistle, or a simple electrical bell. See en.wikipedia.org/wiki/Double_heading. $\endgroup$
    – pereric
    Aug 7, 2023 at 22:22
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Oh, let us have some fun with this...

First, you simply are not going to get the entire route straight and flat - not with engineering of the time, or close. That means there will be sections needing a different sort of engine than for the fast runs. You might switch out just the engine, or switch trains.

Still, there is a lot of fairly flat geography. Given enough(!) human determination, there could be some very long fast stretches.

Also you need to keep the rails very near exact in alignment. Given that ground can shift, this means a need for periodic re-alignment. To my mind this means a simple (hah!) mechanical gadget that runs along the rails, adjusting alignment. This is screws and wedges and a human watching an optical sight, so feasible for the time. (Yes, there would be cost.)

As others had noted, wider gauge is good for stability.

As other posters have noted, there is great advantage to an external power source. But electrifying the entire route seems improbable.

Mechanical gearboxes are going to be a problem - so use electric motors instead. There are motors and generators from that period still in service, so reliability could be good.

This means a steam turbine fitted to a generator, and the wheels driven by electric motors. Efficiency is a concern, but if we imagine rare-earth magnets available early (not a huge stretch), then this is feasible.

The Japanese bullet trains were built in the 1960s, so not huge gap from your time period. Would not be surprised if you could get equivalent 200mph speed.

Train would have a streamlined nose, much like present bullet trains. Powerplant would be a steam turbine, driving an electric generator.

When leaving station most power would come from electrified rails, and perhaps for tens of kilometers after. This reduces peak power requirements, and greatly stretches the on-board fuel. Might be electrified sections in more developed areas.

Likely want something near a closed-loop to reduce water carried. Lots of air flowing past a fast train. Back end of the engine would be huge radiator with aluminum fins parallel to airflow - figure the last 10-20 meters of the engine sides as a stack of fins. (There are similar designs from the period, so not a stretch.)

Could such a thing have been built in that time. Maybe not, but also not so far from possible. :)

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There are a couple of inherent limits to the speed of a steam train, though I've not done the maths to work out the exact numbers.

Firstly, no steam train has ever been fitted with a gear-box, (mainly due to the size/weight that would be required to handle the load). This means that at top speed, the pistons are reciprocating at VERY high cadence - significantly increasing that cadence requires numerous major changes.

Secondly, the very concept of steam power relies on the expansion of the steam, which involves diminishing returns, basically the faster a piston "opens" the lower the force that the steam can exert. And increasing the steam pressure also produces diminishing returns...

To contradict certain other answers, the amount of water needed isn't a major factor. From what I can find, when Mallard did its record-breaking run, it only had seven coaches in tow, which is well below its limit, giving a total of approx 240 tonnes. The tender had a maximum water capacity of 5,000 imp gal / 23,000 L; == 23 Tonnes - less than 10% of total weight.

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