In an area in my world, people have discovered how to turn their aural constructs (magic stuff) into electrical supercapacitors, allowing for relatively advanced electrical technology.

Now that they have the ability to power their trains (and perhaps other vehicles) using electricity, is there any reason they would use electric boilers and steam power rather than directly powering electric motors?

The trains would be powered by on-board supercapacitors that could be exchanged or recharged at select points.

When I say steam-electric, I mean that the energy in the capacitors is run through resistors, heating water and turning it into steam which then moves the wheels using a piston, as opposed to steam being used to generate electricity which powers electric motors.


12 Answers 12


I only provide this answer because no-one else seems to have covered the point that such things have existed in real life; see Wikipedia page on Electric-Steam Locomotives. The page notes that "This is a highly unusual type of locomotive that only makes economic sense under specific conditions."

Those conditions are:

  • An existing fleet of coal-powered steam locomotives.
  • A cheap source hydro-electricity.
  • A shortage of coal.

Presumably the correct convergence of resource availability and shortages could lead to the rise of such technology (again).

In response to any argument that supercapacitors are likely not up to the task, such locomotives "...could run up to 20 minutes without power supply, like a fireless locomotive, once the boiler had been charged to full pressure."

  • 4
    $\begingroup$ real-world answers are often the best! $\endgroup$
    – njzk2
    Apr 24 at 21:31

For train locomotion, straight-electric is vastly superior to electrically-generated-steam in many circumstances.

  • Electric locomotives are much lighter than comparable steam, making infrastructure lighter and cheaper.
  • Electric traction makes distributed driving trucks (multiple-unit operation like subway trains without a locomotive) possible.
  • Electric traction is mechanically simpler than comparable steam. It requires less maintenance.
  • Electric traction is safer: The driver can sit in front with unobscured vision. No exploding boilers. No clouds of steam obscuring signals and fouled track ahead.
  • Electric traction can much more easily match force application to wheel adhesion, preventing wheel-slip when accelerating (and braking, too).
  • Steam traction requires approximately 150 gallons of water per mile ~ 350 litres of water per kilometer (source). The additional watering infrastructure --towers, wells, pumps-- and associated maintenance is unnecessary when using electric traction.

Electrically-generated steam does still have a place:

  • Museums
  • If electric motors are unavailable
  • Certain hazardous industrial settings
  • 3
    $\begingroup$ Also, steam electric requires water. Water was a bigger requirement than coal for steam trains. $\endgroup$
    – Mary
    Apr 22 at 22:08
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    $\begingroup$ @MS Yes. They used to have water towers just to fill up the trains. $\endgroup$
    – Mary
    Apr 23 at 0:14
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    $\begingroup$ Added water requirements of steam to the answer, with source. $\endgroup$
    – user535733
    Apr 23 at 12:56
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    $\begingroup$ Electric engines are in fact so superior that diesel-electric is a thing IRL, where electricity is generated by a diesel generator, but instead of a physical transmission of the energy to the wheels, regular electric motors on the wheels do the actual driving $\endgroup$
    – Hobbamok
    Apr 24 at 8:05
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    $\begingroup$ @Hobbamok Electric motors are great for low-speed traction, whereas IC engines are not - the core advantage is that electric motors don't need a clutch. Steam engines don't need a clutch either, again they have better torque at low speed, so any advantage that electric motors have over steam is a bit more subtle. $\endgroup$
    – MikeB
    Apr 25 at 11:39

Lack of magnetic theory.

If you know about electricity but not about electromagnetism, using electricity to drive a heating element makes some sense.

Generally, it would be surprising if someone developed electrical infrastructure that could support something like trains without noticing that it has some side effects. In your case, it's actually fairly plausible. If magic can generate a big pile of electricity, and magic can bring that electricity to where you want it, most of what you're going to notice first is arcing, sparking, and the heating that accompanies them. So yes: your civ doesn't actually have a coherent electrical theory, they just have a few applications that look much more like lightning-on-demand than like modern electricity.

That probably means your electric lighting is electric arc lighting (fun), and most of the rest of your electric tech is really just steam, generated in a more distributed way than one might usually expect.

  • 1
    $\begingroup$ You can get standard incandescent lighting without magnetic theory. ("If you use electricity to heat metal hot enough, the metal glows!") $\endgroup$
    – R.M.
    Apr 24 at 21:29
  • $\begingroup$ @R.M. Yes. But that's still optimizing for small, predictable amounts of electricity. If your source looks like arcs, I don't think anyone bothers to invent incandescents. Incidentally, arc lamps were a major technology before incandescents took off (example: en.wikipedia.org/wiki/Moonlight_tower ) $\endgroup$
    – fectin
    Apr 24 at 22:51

Nope, no conceivable reason.

The conversion losses would be very bad

It's not the boiler - that would enjoy 100% efficiency, after all. It's the monkeyworks. Classic steam locomotives run around 4-10% efficient, and most of that is not losses in the boiler itself - they are hurt by

  • It takes only 350 BTU to heat water from 62F to 412F, but 970 BTU to make it boil. This "latent heat of vaporization" is the lion's share (almost 3/4) of the energy, and it's largely wasted. Of course, water has a very high LHV compared to other liquids, so a substitute fluid might be an option; or even avoiding the LHV altogether by running it on compressed air. (though I haven't studied in earnest the efficiency of compressing air from battery).
  • Lack of compounding: Efficiency gains require at least 2 stages of expansion, preferably 3 (e.g. the USS Texas's triple expansion engine). That was done on Mallet steam locomotives, but the enormous second stage pistons greatly limit top speed, making it infeasible for other than bulk haulers. Such a slow engine would only "get in the way" on a modern railroad.
  • Absolutely no condensers, so no vacuum being pulled on the low side of the pistons.
  • Lack of any heat recovery to speak of.
  • Poor pulling power performance at low speed, requiring them to keep full pressure steam on the piston face for much of its travel (unable to "notch/hook up"), with that sheer volume of steam largely thrown away.

Mind you, electric traction is nearly as old as railroading itself, surfacing in the 1870s (150 years ago) when railroading had only begun in earnest in 1820s (200 years ago). And traction motor are so ridiculously efficient that self-propelled cars like streetcars, subway and interurbans don't even bother with traction motor blowers. (Locomotives have that.)

As far as batteries, we're almost there today

The supercapacitors would have to be truly magical, as they have profoundly poor energy density compared to batteries. However in batteries, this is practically a solved problem today. They can recharge pretty fast if liquid-cooled. Thin pouch-style batteries particularly lend themselves to liquid cooling, and weight's not a problem on the railroad.

Several light rail vehicles use battery propulsion so they can dispense with overhead wire where impracticable or unaesthetic. They recharge when able to get to wire, slot, or inductor. It's really taking off, since battery power is that good now. Historic trolleys are doing it too.

Unfortunately in the rail industry you'll get a lot of QQ on the idea that a locomotive could ever be battery-electric; the overlap of "people who really grok railroad tech" and "people who really grok battery tech" is very tiny, so almost all commentary on the subject is ill-informed one way or the other. Also, they probably won't ever work for every application; batteries just can't compete with a 30%-efficient modern diesel sitting on top of a 4000 gallon fuel tank. But for many, absolutely. For "grain elevator" level switching locomotives I expect it to happen within the decade.

Wait a minute: there's a chance.

You just need to use a physics cheat code.

OK, so where are you going to get all this heat? Just have arctic cold air blasting out the bottom of the tender? Well, wasn't I just warning about all the utterly wasted heat leaving the steam cylinders?

That normally goes up into the "smoke box" which is about the front 25% of the boiler! Well, the boiler can be a lot smaller when it's interchanging with freon instead of fire, and that leaves room for a smoke box absolutely stuffed to the gills with a multiple-stage heat exchanger.

Yeah. This can bandage virtually all of the above-discussed design faults of the classic steam locomotive, at the expense of maintaining a bunch of refrigeration plant running on a variety of different refrigerants.

And thanks to the magic of having the tender mysteriously blasting freakishly cold air, we can actually run all this plant somewhat over-unity. But that's only over-unity thermally; there's no free lunch, and we can't beat traction motor efficiency from battery to drawbar. But we can at least recoup the vast majority of steam locomotive wastage, and maybe get close enough that cultural factors could influence the choice.

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    $\begingroup$ There's also the matter of torque and controlling it, where electric is outstanding, that's why even a lot of "diesel" locomotives are in fact diesel-electric. $\endgroup$
    – biziclop
    Apr 23 at 0:26
  • $\begingroup$ @biziclop "a lot"? as in almost all! $\endgroup$
    – Glen Yates
    Apr 24 at 21:40
  • $\begingroup$ @GlenYates That's a lot in my book :) $\endgroup$
    – biziclop
    Apr 24 at 21:49
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    $\begingroup$ The latent head of vaporisation isn't wasted. Steam engines have condensers, so you get it back again. Specifically they use the Rankine cycle, which gets more efficient when the working fluid has a higher latent heat, not lower. They do vent some steam, but that's part of a mechanism for increasing airflow over the fire. $\endgroup$
    – N. Virgo
    Apr 25 at 1:37
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    $\begingroup$ @N.Virgo Well I don't see any reason a piston plant (e.g. USS Texas) couldn't use condensers., though that would be less useful if you're not compound/multi-stage. It really helps to have an ultimate heat sink, like a sea chest or a cooling pond. That's the problem on a steam loco, nowhere to put a condenser that large! $\endgroup$ Apr 25 at 18:31

Nobody in Your World Has Created a Good Electric Motor Yet

Perhaps nobody has invented an electric motor yet, or, if they have, they still have limitations that make them impractical or unsafe to use for these applications. Perhaps in your story world, they already have invented good electric heating elements that are already being used in other commercial equipment to power steam engines. Perhaps they’re even using steam turbines. Before the AC brushless motor invented by Nikola Tesla, competition between brushed motors and steam was actually still somewhat stiff, because brushed motors at the time had several limitations. (Also the power grid was still not very widespread yet). But that’s my answer for you, the electric motors in your world are still currently very limited, and can’t compete with steam power yet.

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    $\begingroup$ This is also my thought. Some of our best motors use exotic materials for the permanent magnets (rare earths and such). If they weren't available, motors would be less powerful. Motors exist without permanent magnets but they tend to be less attractive for traction. $\endgroup$
    – hughk
    Apr 24 at 9:56
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    $\begingroup$ @hughk why should motors without permanent magnets not be used? A universal motor, for example, naively seems like a reasonable choice. Certainly better than a steam engine - lower maintenance, higher efficiency, doesn't need water. $\endgroup$
    – user253751
    Apr 24 at 12:40
  • $\begingroup$ Perm magnet motors are found in many vehicles today as they offer size, weight and cost benefits at a given power level. Size/weight isn't so much an issue on trains but I was trying to find a reason not to use electricity for motion without turning to magic. $\endgroup$
    – hughk
    Apr 27 at 7:18

Conspicuous consumption and sexual competition.

The clouds of steam that the trains give off prove the magical ability of the person running it. These clouds can be made to have multiple colors further showing magical strength. Doing such a show attracts the females better and deters other males.


Given you're already talking about magitech, you're already shifting out of real world physics and economics. Others have pointed out that the basic efficiency of a steam engine isn't great, but you have some latitude to tweak things a little. Anyway, here's a few suggestions that may be initially plausible enough even if they wouldn't win in this world:

  • The material components of an electric motor, such as rare earth metals for magnets, are rarer than they are on Earth, or indeed they're just as common but there's fiercer competition to use them in other applications.
  • Magic makes electrical energy "too cheap to meter". This relates to the above, but basically we're suggesting that energy efficiency is no longer a core consideration compared to other costs.
  • There are government incentives because of positive externalities. Most of the railway track runs through a natural desert and artificially dumping water into the sky is helpful for preventing the dunes from taking over again. You might think that irrigation pipes would do the same much more efficiently, but it's a larger infrastructure cost and wouldn't pay off until a future election cycle.
  • There is a national security value in simplicity. The Fey may have generously provided the use of magic, but the government doesn't entirely trust them. It is therefore policy that, as rail is a key piece of militarily significant infrastructure, any engineer must be able to grab her wrench and an axe to turn a magitech train into a wood-fueled train should there be an unexpected diplomatic issue with the forest folk.
  • In a similar vein and going back to short termist thinking, the infrastructure already exists. There is a monopoly which owns all the track and runs the trains. It's easier to retrofit a wood boiler based train into a magic boiler based train than into a magi-electric motor based train, and it helps with the profits for the next shareholder meeting. To make matters worse, a competitor holds a patent over the magi-electric motor and they do not want to negotiate favourably.
  • Your magic is much better at creating chaotic energy than orderly energy. Perhaps it gives unpredicatable high frequency short terms bursts of electric energy, and the circuitry to regularize that energy cannot handle the power loads involved in driving a train. A big boiler which just wants heat is a very scalable system for using such chaotic inputs.
  • It's a steam train, but not water steam. As Harper pointed out, water is a marvelous material but its latent heat of vaporization costs you a huge proportion of your energy. Purely for example, an Acetic acid (vinegar) train would have to contend with just one fifth of that. Of course you'd have to explain away why having hot acidic gas in your pipes doesn't destroy them and the surrounding countryside, or make it a plot point that it does! Alternatively, pick some other common-ish fluid. This table may help.

As always, the key question is going to be what makes sense in your story and for your goal. If you were going for the aesthetic of steam train chimneys, you don't want something that recycles your vinegar instead of venting it. If you want a chaotic world, you don't want effective government intervention.

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    $\begingroup$ The latent heat isn't wasted - a high latent heat is actually a good thing for a heat engine using the Rankine cycle. $\endgroup$
    – N. Virgo
    Apr 25 at 1:40
  • $\begingroup$ That's a good point. I had the impression (and to be clear, I am nothing resembling a locomotive engineer!) that the sort of engine found in a traditional "Choo Choo" steam train vented the steam, which would mean the energy is necessarily lost. I agree that the story would be different if there were additional machinery to recover it. That goes back to my final question of what aspect of steam trains are helpful for the story and therefore how different the design could be. $\endgroup$
    – Josiah
    Apr 25 at 7:20
  • $\begingroup$ I'm not a locomotive expert either (I'm more of a theoretical thermodynamics person) but I think they only vent a small amount of steam on each cycle. It's used to increase airflow over the fire, but there might be some other reason for venting it as well - I don't know. $\endgroup$
    – N. Virgo
    Apr 25 at 8:04
  • $\begingroup$ Actually I think I might have been wrong about that. Reading up on the history of locomotives it seems that early designs (like Watt's) work how I thought they did, but later ones do actually just vent the steam into the atmosphere at high pressure to avoid the weight and complexity of a condenser. I'm unsure whether that means the latent heat is wasted or not. $\endgroup$
    – N. Virgo
    Apr 25 at 8:14

Regulatory compliance.

You think the real world is the only world with stupid regulatory requirements? There is no reason why a magic world shouldn't be the same. The steam engine companies, unions and operating companies are a vast lobby group with a lock on government, because of "safety", "economics", "nature" and "our traditional way of life".

CF "butter mountain", "Big Sugar", and "Interstate Commerce Commission"


On steam-electric

Steam-electric is what you'd call an electric motor powered by a steam engine (which isn't what the question is about, but it's still interesting to talk about that). This is based on diesel-electric trains, which are just that but with diesel.

Diesel-electric trains are the norm for diesel trains these days, they indeed run a diesel engine that powers electric motors. They're the norm because they're generally less complex. Primarily you don't have to transmit power mechanically and you don't need a complicated gearbox.

For a modern train, there's also the advantage of being compatible with a fully electric powertrain, and this allows bi-mode (what you'd call hybrid for a car) trains that can run on electricity or diesel relatively easily. Diesel-electric or bi-mode trains are quite useful in area that haven't been electrified.

Steam-electric trains would be an alternative in a world without diesel. And I insist on a "world without diesel" part, because steam was replaced by diesel for a reason: it's a lot more efficient. Not as much as purely electric (there's a reason diesel has been largely replaced by electricity), but certainly more than steam.

You would have steam-electric trains because the rail network isn't completely electrified, and some areas are particular hard to electrify (remote areas, mountains) or just not economic to electrify (low traffic). But wherever you have reliable electrified lines, they would likely be edged out by purely electric or bi-mode trains.

On electric-steam

Electric-steam I suppose is what you'd call a steam engine powered by electricity (which is what the question is about).

You could reverse the process to use electricity to boil water for your steam engine. Boiling water with electricity is common in home appliances for cooking or tea. However, doing it to power a steam engine is absurd.

Why? Steam engines aren't very efficient at all. Steam locomotives of yore usually had single-digit energy efficiency, and even modern steam turbines used in power plants today only reach about 40%. Comparatively, a good electric motor will have double that.

You would need absurdly efficient steam engines and absurdly inefficient or inexistant electric motors to justify electric-steam traction over steam-electric or electric traction.

Electric traction + unrelated steam

If electricy is magic and comes without power lines, there's not a good use case for anything but electric traction. If it comes from power lines, steam-electric might make sense for lines without reliable access to electricity.

Any other use case or electric-steam traction would generally be a waste of energy when you could just use the electricity directly.

The only reason to have a steam engine would be if you needed the steam for something else. Maybe you're running a luxury spa-train with steam rooms. I don't know what else you could use steam for (that electricity wouldn't do a better job at) on a train, but that's certainly one use.

Even then, the traction would still be fully electric, and you'd have a separate system that converts electricity into steam.

  • $\begingroup$ +1 for "if you needed the steam for something else". That's really the only way to make this reasonable. If you were in an incredibly cold environment, re-using that steam to heat the train interior might be more efficient than an electric train with electric heating. $\endgroup$
    – bta
    Apr 25 at 22:48

In a niche application, maybe.

In our world, some chemical plants - say, producing explosives, used fireless locomotives. These periodically fill up their "boiler" with superheated water, which flashes to steam to produce power for several hours operation.

The filling operation (and the boiler, which could be electric or even a relatively efficient heat pump) is off site; then the locomotive works on site without the danger of fire ... or electric sparks ... igniting the product and destroying the neighbourhood.

But generally, outside the writings of Flann o'Brien there is no other role for an electric heated steam locomotive. It's just too much waste of power.


It's a stretch, but

The Wrong Kind of Electricity

Your magical batteries output electricity at a very high voltage, with alternating or unpredictable current. Your people lack the technology to rectify it into the few hundred volts DC preferred by traction motors, or they have it, but they cannot miniaturise it enough to be carried around on a train.

A heating element is just a coil of wire and won't much mind whatever weird electricity you throw at it. It may be hilariously inefficient, but it's still cheaper than coal.


Rube Goldberg is the Emperor of the Universe

Per imperial decree, every mechanical device in your world must achieve its purpose in the most complicated, convoluted, and indirect way possible.

As a result, the usual way to accomplish anything in your world looks like this:

enter image description here

(Source: The Incredible Machine, a classic game built around the idea)


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