Assume in the very near future almost all rail transportation would return to steam-powered locomotives. To define it exactly, I mean solid fuel is burned inside the locomotive, which boils water, which is in turn used to drive the locomotive.

Assume no economic or societal collapse, or at least not a too large one. Also assume that the importance of rail transport versus road transport increases to the levels it was 70-90 years ago. Technology should be the same as we have now.

Steam locomotives are not very efficient, but there might be some causes in their re-emergence. For example, rising oil prices would make road travel uneconomical for goods and only affordable for personal travel to the wealthy, just like it was 70-90 years ago. Or some regulations would be the cause of this. Whatever the reason, steam locomotives are coming back.

(I know that this would explain the reduction in diesel locomotives, but not electrical ones, which can be powered by wind/solar/nuclear power. Maybe something like a new plant which grows fast, easy to harvest, and burns well, perfect for steam locomotives but not economical for power plants? I know, getting rid of electrified rail would not be easy, this might require some suspension of disbelief. But somehow, they are either gone, or reduced to niche roles)

Now, steam locomotives are back, while technology is roughly the current one. What would they look like?

enter image description here

Steam locomotives, especially 20th century ones, are real marvels of engineering, more than 100 years of development led them to be much faster and more efficient than when they were first invented. This knowledge is not lost to us, so we don't have to start from scratch. Many old steam locomotives still exist in functional states with people able to maintain and operate them, either for tourism, or in reserve for emergencies. So, putting the 1930-1940's steam locomotives (the last time significant development was done in this regard) back in production should not be that difficult. But how could we improve them?

One significant change (but invisible from the outside), I guess, would be the computerization of the controls, to make them simpler to operate. Driving a steam locomotive was a very difficult job, and required great wisdom. It's no wonder the word "engineer" originally meant a train driver. Even the job of the person shoveling coal required a lot of knowledge and experience, keeping track of steam pressure, temperature, humidity, the color of the smoke, to know when to shovel fuel and when to release pressure, etc. Everything was done manually, so some improvement could be done there.

What else? Fuel efficiency? What could we do now what they couldn't do 70 years ago? Computer simulations to make a better shape for the boiler to improve efficiency a little? Better steel technology to resist higher pressures? Better filters to not produce that much smoke?

What would visibly change as steam locomotives would be kept improving from where they were left 70 years ago? Steam turbines are more efficient then traditional steam engines, every power plant, including nuclear ones, use highly sophisticated steam turbines. However, they tried it for locomotives, and it was not very successful.

  • $\begingroup$ Why is this a hard science question? The question is asking for speculation, not science. And none of the answers(which the op has complimented) meet the hard science requirements. $\endgroup$ – user171 Aug 16 '16 at 16:22
  • $\begingroup$ @Hamlet : what exactly did I compliment? On both answers I actually raised issues about them. And indeed, I was asking about requirements and how to solve them. For example, the problem of steam turbines is that they are only efficient at a very exact rpm, so good for a power station or a ship, not so good for a locomotive. Geared transmission is generally not used in locomotives, especially in bigger ones, and there must be a reason why. However, steam locomotives were not improved upon during the last 70 years mainly because other types were more profitable, so there is plenty to explore. $\endgroup$ – vsz Aug 16 '16 at 17:26
  • $\begingroup$ the problem here is that your asking us to predict the future using science. That can't be done: predicting the future is speculation, not A science if you edited this question to focus on the various possible ways steam engines could improve (As opposed to asking us to predict the future), and if you defined how steam engines should improve (eg fuel efficiency) then I wouldn't have this objection $\endgroup$ – user171 Aug 16 '16 at 21:50
  • $\begingroup$ Form follows function. Why shouldn’t they look like a steqm engine on wheels? $\endgroup$ – JDługosz Aug 16 '16 at 22:23
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    $\begingroup$ steam power is a main driver in some countries, e.g. France: The steam engine is connected to a nuclear power plant. $\endgroup$ – mart Aug 17 '16 at 7:44

11 Answers 11

  • Most steam engines would use condensers, both to avoid watering stops and to use the thermal energy to pre-heat the feed water.
  • Many steam engines would use modern gearing rather than pistons and rods directly coupled to the wheels. This allows the pistons to turn at the optimal speed for power generation and not at the necessary speed to turn the wheels. That would mean driving wheels can get smaller, too, no more need for the outsize wheels of old locomotives.
  • Some of them might use steam turbine technology.
  • You might see steam-electric engines where the steam engine drives a generator and that generator powers electric motors.
  • Technology might be developed to replace the stoker who hauls chunks of coal with a pipeline to transport coal dust.
  • $\begingroup$ About the gearing: I guess there is a reason why they are not used in anything larger than small maintenance vehicles. The technology was known in the steam age (they had cars back then!). Also, I never heard about any large diesel locomotive with a geared transmission, they use either hydraulics or an electrical generator. $\endgroup$ – vsz Aug 16 '16 at 16:04
  • $\begingroup$ @vsz, that will depend on the quality of the gears: efficiency, durability, etc. I would like to think that has gotten better during the last century. $\endgroup$ – o.m. Aug 17 '16 at 4:46
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    $\begingroup$ @o.m., You have to remember that the Victorians were really good engineers. A modern computer modeled ship propeller is only 5% more efficient than the one Brunel put on SS Great Britain. $\endgroup$ – Separatrix Aug 17 '16 at 10:04
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    $\begingroup$ Actually, modern gearing is better. The problem was not engineering, it was metallurgy. We have better alloys for high power gearings. Not sure if a steam engine is powerful enough for this to be relevant, but the history of pre-WW2 tanks and warships is full of complaints about gearing reliability so... probably. $\endgroup$ – Ville Niemi Dec 6 '17 at 3:03

It will not happen

You asked:

What could we do now what they couldn't do 70 years ago?

Answer: they could not create an electrified railway for locomotives to use instead of lugging all their fuel around.

When it comes to generating power, there is significant economy in scale and centralization. If diesel-electric engines become nonviable, this will not bring back steam engines. It will instead lead to a boom in electrical engines. The power you wish to generate on-board the train will not be done so, but instead at a power plant. As far as economics, efficiency and logistics are concerned, this is the best way to go, if nothing else because it means we can use already existing technology instead of having to re-invent the steam engine in the 21st century.

Also there is no more efficient way to distributing energy than as electricity. Lugging coal and water — or any other steam forming agent around — is horrendously tiresome.

If you are hellbent on creating steam engines again, they will not be the stereotypical puffing chugga-chugga-chugga kind that we are used to, but instead just versions of the diesel-electric kind, where the steam engine drives a generator that in turn drives electrical motors on the wheels. So they will be quite boring looking, and not the stylish art deco machines of the 1920s to 1940s.

I am sorry, but since you asked for "hard science" I am forced to give you a boring answer.

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    $\begingroup$ Hmm, interesting - first I thought "why would turning chemical energy into kinetic, then into electric and back to kinetic be preferrable", but since diesel-electric engines exist there had to be a reason. Turns out transmitting large amounts of kinetic energy from your engine to your wheels over large-ish distances sucks because it puts a lot of mechanical strain on the parts involved, right down (or up, or whichever direction^^) to the frame. So take your +1 for a good sciency answer :) $\endgroup$ – Syndic Aug 18 '16 at 13:42
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    $\begingroup$ @Syndic The reason is engine efficiency. Engines, especially all kinds of expansion engines, operate at their peak efficiency within a narrow regimen when it comes to power output and RPM. There is only one kind of engine that is "happy" at pretty much any RPM and power output, and that is the electrical engine. Using expansion engines for direct drive of wheels is very inefficient and also demands you add gearboxes, clutches and other such devices, which are prone to breakdowns and require maintenance. Hence the need for hybrid engines; petrol/electric or — in this case — steam/electric. $\endgroup$ – MichaelK Aug 18 '16 at 13:54
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    $\begingroup$ @Syndic With a hybrid engine you can run the expansion engine at its peak efficiency at all times, and deal with variations in power output by having an energy buffer — i.e. a battery — between the generator and the electrical drive-engines. Also, since electricity can easily be transported from a power plant to any point on a railway track, it is entirely unnecessary to lug the steam expansion engine and the generator along... you leave those at your power plant. $\endgroup$ – MichaelK Aug 18 '16 at 13:57
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    $\begingroup$ Part of the reason the US doesn't use electric everywhere is the logistics of building an maintaining substations and all that catenary everywhere -- we're a rather large nation with vast expanses of open land that need railroad tracks through them. Even if you couldn't use diesel for some reason, it could still be more economical to tote around steam-electric generators than build and maintain all that overhead wiring, at least in the US. $\endgroup$ – zstewart Aug 18 '16 at 22:35
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    $\begingroup$ What's "economical" isn't static, it changes as technologies and resources change. Electricity transfer and generation gets cheaper and cheaper, since it's needed for everything else. Steam engines, not so much. On the other hand, maintaining a substation is always cheaper than maintaining a refuel station --- and a substation can provide power for a road town as a side benefit. Also, as someone who rode electric trains from Moscow to Vladivostok, I scoff at your "vast expanses". $\endgroup$ – Alice Dec 6 '17 at 11:48

From a purely aesthetic point of view, I hope they come back as art deco beauties

art deco train

An example of this is PRR-S1

art deco train 2

Also, you could make them NUCLEAR! Using RTG's like in satellites, no pollution at all! With a continuous heat source, you have no coal man or fuel to transport, just a lot of water.

  • $\begingroup$ Perfect answer. In fact, if the security issues with nuclear power ever to be solved, this would be the future of trains. But I would imagine a more modern look. $\endgroup$ – Cem Kalyoncu Aug 16 '16 at 15:31
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    $\begingroup$ With the current hype against anything nuclear, I guess there would be a lot of opposition, up to outright demonstrations or blockades against it. There are lots of activists who would like nuclear power to be banned completely, usually stemming from misinformation. $\endgroup$ – vsz Aug 16 '16 at 15:35
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    $\begingroup$ @CemKalyoncu I understand your opinion on the looks but I am in love with art deco $\endgroup$ – Chris J Aug 16 '16 at 15:36
  • $\begingroup$ @vsz I think it depends on how its sold, RTGs are not the same as a nuclear power station and with the right spin it could be done... I can't find a source for when an RTG has failed $\endgroup$ – Chris J Aug 16 '16 at 15:39
  • $\begingroup$ Do you think an RTG that produces enough power to "push a train" is feasible? Most current steam powered ships still use the mechanical power of steam for propulsion, not electrical power. They also generate electrical power, but the screw is driven by a giant reduction gear. $\endgroup$ – Seeds Aug 16 '16 at 16:26

Steam engines went out of fashion because they have significant issues compared to diesel electric engines.

  1. The power to weight ratio is rather low
  2. Steam engines can take up to 10 hours to raise steam from a cold start
  3. The weight of water,fuel etc. is far greater than the corresponding weight of diesel fuel
  4. Steam engines are relatively complex machines with significant safety issues from the use of live steam and a pressurized boiler.

So to bring back a steam engine, we would have to find ways to overcome these issues. Some of the issues could actually be resolved through the use of supercritical CO2 as the medium of energy transfer other than water. The higher density allows far less fluid to be used, the energy efficiency is much higher and since there is a smaller amount of fluid in the system, the "raise steam" time would be much reduced. Experiments using supercritical CO2 rather than steam in thermal power plants suggest that a turbine and the associated plumbing could be 1/10 the size of the equivalent steam turbine.




The obvious result would be the space devoted to the "engine" would be far smaller, and since this is a closed cycle turbine system, the resulting engine would be smaller and lighter as well. This would be offset somewhat by the need for large radiators, and of course the issue of highly pressurized working fluid hasn't been eliminated either.

Direct drive from the turbine to the drive wheels isn't the best solution (the reduction gear case would offset much of the size and weight advantages of using a supercritical CO2 turbine), so the turbine would be powering a high speed generator. A steam-electric serial drive would operate in a similar fashion to current diesel electric engines. Because you are using a turbine, this potentially allows you to produce the same energy output from one locomotive as several diesel electric locomotives. This could be tapped by providing powered bogies on the railcars themselves, allowing for distributed power along the length of the train and potentially providing more traction for starts, stops, moving up steep grades and so on.

The other advantage for the railway company is they would only need one locomotive per train which would be an especially useful cost saving when running large and heavy unit trains

enter image description here

  • $\begingroup$ I agree with all what you wrote, but there is one problem: in the fictional scenario something caused steam locomotives to become common again. This obviously didn't happen in real life, so in all what you wrote there is no point of divergence. Still, it's a pretty realistic scenario what you presented. And actually, most people would not even notice it, because the new steam turbine locomotives would look (visually, from the outside) very similar to modern diesel locomotives. What I was looking for was to go back to the retro style steam locomotives and start adding improvements to them. $\endgroup$ – vsz Aug 16 '16 at 20:05
  • $\begingroup$ One possible offset to the weight of water could be methods of reclaiming "used" steam by condensers. That would depend on if the weight of a condenser large enough to reuse most of the water was significantly less than hauling additional water. $\endgroup$ – Michael Richardson Feb 9 '18 at 20:16

If you look at a late-era steam locomotive like the Mallard, I think there are real similarities to more modern trains, e.g.:

enter image description here enter image description here

I would expect train manufactuers to continue to design streamlined locomotives that both have good aerodynamic properties and evoke ideas of speed and modernism.

They probably wouldn't be white because white stuff gets dirty around coal. Dark colours are more likely.

I would expect the driver to be at the front because (a) most of the business of fueling the train would be handled by automated machines, (b) the controls would be electronic rather than mechanical and could be operated from a remote cabin, and (c) the front offers better forward visibility and safety.

The coal tender would be covered rather than open for various reasons. Firstly, it would again help the locomotive to look more modern and streamlined. Secondly, there may be some modular solution for more efficient refueling (rather than just 'pouring coal in from the top'). Thirdly, refinements in fuel chemistry and the technology that transports the fuel into the firebox mean that we need to protect the fuel from the elements.

For long-distance trains, there may be solutions for storing fuel above/below the carriages and then automatically transporting it forward to the ending as needed.

Having said all that, here is some cool art called "Heavy Metal Hero" by Rodney Matthews:

enter image description here

This art was also used on the cover of an old videogame called Transarctica, set in a post-apocolyptic frozen world where people live on trains that roam the landscape looking for supplies.

  • $\begingroup$ I upvoted your answer just for that third locomotive. (Rule of Cool.) $\endgroup$ – Codes with Hammer Mar 26 '19 at 14:06
  • $\begingroup$ I upvoted it for the first one - the Mallard still holds the world speed record for steam locomotives! $\endgroup$ – Aaron F Nov 26 '19 at 17:37

Steam power would be steam-electric

Diesel trains don't run diesel engines coupled to gearboxes coupled to the wheels. Modern ones will use a diesel to power a generator to power a VVVF (Variable-voltage, variable-frequency) drive unit. The electric generator and motor act as the transmission. This is more efficient today; if you change the prime mover, the generator/drive combination is unaffected and will still be more effective (namely, lighter) than a gearbox mechanical transmission.

For electric generation, a turbine is best

Since you are now connected to an electric generator, the most stable and efficient way to operate one of those is with a constant speed turbine. What do you know, the same technology setup is still used on to the modern day in steam powerplants on warships. The Nimitz aircraft carrier runs (many) 8,000 kW steam turbines, a locomotive might be more like 2,000 kW.

Does it really have to be steam?

You know, if you are going to run a turbine-electric system on a train, why not make it a gas turbine? This is basically a jet engine; instead of putting steam in to power the turbine, you put gasoline in and light it on fire. The power to weight ratio of those things are off the charts (compared to both diesel and steam), and the turbine output is basically the same as from a steam turbine.

  • $\begingroup$ Locomotive power really depends on where you run them. But as a rule of thumb you'd rather go up to 3MW instead of 2MW $\endgroup$ – dot_Sp0T Dec 21 '17 at 15:32
  • $\begingroup$ As I commented on another answer, my second thought about this question was a steam-electric locomotive. $\endgroup$ – Codes with Hammer Mar 26 '19 at 14:08

The other commenters have done a good job pointing out what might change with the braking and mechanical control systems of a modern steam locomotive would be, as well as the engineering reasons steam lost favor to begin with. However in the the middle of the 20th century French, Argentinean, and British engineers worked hard to radically improve the performance of steam locomotives. Some of the last results looked like this: enter image description here

It's close to what I imagined when I saw your post, as this is the world building stack exchange after all. The main innovation beyond aesthetics is the thermodynamics of the boiler and fuel feed system. It requires a some basic thermo knowledge to appreciate, but is detailed very well here: https://csrail.org/modern-steam

A fully report on the sleek train pictured is here: https://static1.squarespace.com/static/55e5ef3fe4b0d3b9ddaa5954/t/55e637bee4b0bef289260255/1441150910433/%23+DOMS-2_PORTA_Argentina.pdf

Almost everything you are looking for is linked to from this wikipedia page from a prototype steam locomotive designed in the 1990s.https://en.wikipedia.org/wiki/5AT_Advanced_Technology_Steam_Locomotive

Engineers continue to refurbish and build new steam locomotives today, but most are straightforward rebuilds of midcentury models with the most minimal changes to make them meet safety parameters. They retain the same essentially "non-modern" body you'd imagine.

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    $\begingroup$ We prefer that you put the relevant part of those links in the answer. In this way, if the links for any reason no longer work, the information supporting your answer will still be maintained. $\endgroup$ – L.Dutch - Reinstate Monica Feb 19 '19 at 6:49

Why does it have to run on solid fuel? Fluids are easier to handle. Steam locomotives have traditionally run on poor quality oil. Recent examples are tho new and converted locomotives from DLM which run on light oil or diesel fuel. However, steam locomotives might use biowaste such as wood chippings or agricultural waste such as straw, though the energy density per unit volume is very low.

Apart from that, there is no reason why they should look much different from the locomotives of the late 1940s. It would be desirable if they could run equally well in either direction, which is primarily a matter of the design of the cab and fuel-and-water-carrying vehicle. Very large freight locomotives would, with advantage, follow the Beyer-Garrett configuration.

The great advantage of the conventional steam locomotive is that it is simple, with a relatively small number of components, of which the wearing parts can be designed for easy replacement. Also, in the railway environment, the boiler acts as an energy reservoir, providing a "buffer" between conversion of the chemical energy of the fuel into the potential energy in the steam, and thence to mechanical energy via the direct drive.

Boiler wear can be substantially reduced by using liquid fuel and effective water treatment at very high pH, and by keeping the machines permanently warm to avoid heating/cooling cycles. External combustion enables the locomotives to be designed so as to minimise the production of harmful wastes such as carbon monoxide, NOx and particulates.

The long preparation times referred to by some of the above commentators can be eliminated by internal or external pre-heating systems using oil, gas or off-peak electricity.

In my view, it is time that this technology made a comeback at least on secondary routes where the density of traffic did not justify the capital costs of electrification.

Overall thermal efficiency these days is about 12% which is not very good, but waste heat can be used for heating the train. It is worth remembering that the most powerful locomotives in Britain are the class 68 diesel and the Duke of Gloucester, the last British Railways steam design.


I'd wager a modern steamer would look something like this, maybe with an added streamlining shroud and bigger. Mainly because high pressure engines were found to be efficient and very successful, but were still premature during the steam era. When high pressure boilers were more perfected, the steam age had long died, and this prototype of mass potential never came to fruition.

  • $\begingroup$ There actually was a "jet train" once, but not quite what you are describing: gizmodo.com/… $\endgroup$ – Thucydides Mar 18 '18 at 6:18

There are 38 new steam locomotives currently under construction in the UK designed to be fired by a range of fuels from solids, liquids and even gas, all of which follow traditional 1930's and 40's configuration and layout. The largest and most powerful of these, the P2, is expected to be capable of producing 3000hp at 80% and 100mph. It's worth noting that during WWII Switzerland, finding it's self cut off from German coal supplies and with a surplus of hydro-electric power it could no longer sell to Germany, converted existing steam locomotives to use immersion heaters supplied via pantographs fitted to the footplate roof. The main problems I can see with steam over electric is power and speed. Steam tops out at a power of around 4000hp for a practical size of locomotive compared with a similar size electric locomotive able to produce 10,000hp. The UK's Tornado steam locomotive built in 2009 has managed 116mph flat out, 109mph slower than the designed top speed of the electric high speed trains using the the same line


Steam describes the medium for translating heat energy into mechanical energy. Nuclear power is often actually steam turbines heated by fissionable material. We'd probably call it a nuclear or atomic train, but it is still a steam engine.


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