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So, context: This is some time after the apocalyptic event, and society has had a little time to re-build. The apocalyptic event in question physically destroyed a large number of cities, and killed most of the human population. It did not poison the air, or make the surface world uninhabitable. While much of this world is now desert, most of the desert predates the apocalypse.

There is very little coal in this world; as a result, steam locomotives have historically been oil burners. At some point in the past, oil refining was discovered, and diesel-electric locomotives became the dominant form of rail transport. (bonus question: with the majority of locomotives using oil rather than coal, how much faster would diesel locomotives have overtaken steam?)

Considering that society has recovered to the point that trains in general have become not only possible, but worth the time and effort, how likely is it that this society is already able to refine oil to the extent needed to run diesel-electric locomotives?

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    $\begingroup$ Diesel engines do not need "refined" fuel. (Or, more correctly, Diesel engines can be made to run on just about anything that burns, from coal dust to unrefined heavy petroleum to sunflower / canola / olive oil. Of course, this doesn't apply to the modern common-rail computer-controlled smelling-like-a-rose small Diesel engines used in Audis and so on. But the really big Diesel engines used to propel ships and locomotives are very tolerat of what is put in the tank.) $\endgroup$ – AlexP Feb 21 at 8:29
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    $\begingroup$ If the options are, thing that doesn't move if it doesn't work, and thing that blows up if you don't know how to work it, I'll take the former. Developed in the late 30s, it took about ten years. "Considering that society has recovered to the point that" it's about the year 1900, you have about half a decade until they're completely phased out. The last paragraph is unnecessary, and circularly puts you into a semantic 100% probability. $\endgroup$ – Mazura Feb 21 at 11:58
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    $\begingroup$ You have a bigger problem, upkeep and production of rail lines require quite a large investments in upkeep, if your civilization can't refine fuel they don't have the manpower for such upkeep. $\endgroup$ – John Feb 21 at 17:12
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    $\begingroup$ You're making a false dichotomy between diesel and steam. Steam can run just fine on diesel. Your choice should be stated as between diesel-electric and steam. $\endgroup$ – NomadMaker Feb 21 at 18:45
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    $\begingroup$ Never mind coal, historically in our world a lot of US engines were wood burning. If you've not got that then @Mazura 's point becomes a lot more acute and a "largely desert" world can never achieve current-tech levels. $\endgroup$ – pjc50 Feb 22 at 12:14

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Depends on how much infrastructure survives:

So diesel engines can run on a lot of things:

  • Blue Crude
    • water + electrolysis + CO2 + Fischer-Tropsch process == blue crude. You can run an engine on this directly apparently.
  • Vegetable oils.
    • Used for cooking and unused.
  • Biodiesel:
    • Processed crops. Ie Rapeseed (canola crop). Palm oil. Any crop really.
    • Also animal fats
  • Some experimental algaes can make diesel fuel.

If you're wanting to stretch diesel fuel, you can add Ethanol or Butanol. Ethanol you can make post-apocalypse quite easily. You can stretch it even further and run with even lower quality fuel if your train has a fuel pre-heater.

So just because you don't have big oil companies anymore doesn't mean your diesel engines are useless. Blue crude will need power, water, and an intact processing facility. Others will need farmland and some processing, which can be as simple as a press and fabric for filtering.

To answer your question directly, if society is totally gone, steam engine wins. However if there is pockets of surviving industrial facilities or agriculture then I wouldn't write diesel off unconditionally.

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    $\begingroup$ electrolysis - where energy comes from $\endgroup$ – MolbOrg Feb 21 at 7:08
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    $\begingroup$ Have you ever seen a disassembled steam engine? Steam-powered trains are a product of the industrial revolution. If society is gone, they are WAY too complicated to build. I agree with your answer (great analysis of fuels), but not with that line. $\endgroup$ – DWKraus Feb 21 at 13:09
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    $\begingroup$ @DWKraus no-one is saying steam is trivial. The question is between Steam locomotive for train, or Diesel-Electric locomotive for train. Diesel is way, way, more demanding on high-tech processes, and to then also include ultrahigh-amperage electric generators and motors? $\endgroup$ – PcMan Feb 21 at 15:10
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Take a trip to your local train museum:

To get a good idea about this, go to your local train museum. Here in Minnesota, we have a long history of trains, and I have old train maintenance facilities right here in the Twin cites. Find one that's restoring a steam locomotive. They SOUND simple, but the engineering is only "simple" because it's older. There are hundreds of miles of pipes, thousands of joins, and vast amounts of engineering that go into steam engines. They are VERY MUCH dependent on a large and integrated industrial complex to even function. The maintenance was the real killer with such complicated machinery. Diesel won out because it was simpler to use diesel to make electricity that actually runs trains. Steam relied on mechanical propulsion, which is actually MUCH harder from an engineering standpoint (sorry steampunk folks).

Rails are just a physical means of having a better road with less resistance to carry really large loads (and large machines to pull those loads). You won't have railroads in a post-apocalyptic society without a fair amount of infrastructure. But Diesel engines with have a lot of versatile applications, while steam is likely to be relegated to electrical grid generation like it is today. Redeveloping Diesel means both electrical generation, but also trucks, cars, trains, small boats and all that comes with them.

So in a world of limited resources, you'd start building trains with Diesel using existing engines and tracks, but then you'd CONTINUE using Diesel because it's ultimately simpler, requires less maintenance, with known engineering that applies across a large number of applications.

  • PS in a desert world, I wholeheartedly agree with user6760's assessment that the shortage of water (and likely wood, the easiest source of fuel for early-model trains) due to desertification will be as much or more of a barrier to the use of steam engines as the engineering. Diesel won't require the constant refilling with water that steam engines require. And if wood isn't critical for fuel (as trees are NOT abundant in desert conditions) then the existing oil-based fuels will already require an oil infrastructure that lends itself well to Diesel engines. Diesel fuel is relatively easy to make from crude oil, and can be even made today by people in their own homes from cooking oil. I inherited a piece of land in Kansas with very low-grade oil, and if I wanted to, I could make Diesel fuel from it (I have a chemistry degree, but it's not impossible). As Steven Klassen pointed out, fuel isn't a big issue.
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    $\begingroup$ "Find one that's restoring a steam locomotive." Ah, but that's EXACTLY why I expect a post-apoc society to use steam. Find a museum that's restoring a really early diesel locomotive. There aren't a lot of those. Why? Because, due to lower-precision machining, a steam locomotive shop can make its own spare parts, while a diesel shop is largely limited to installing and servicing spare parts from a factory. Since the earliest diesels are long out of regular service, there are often no aftermarket spare parts suppliers remaining... $\endgroup$ – Tristan Klassen Feb 21 at 15:44
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    $\begingroup$ The issue, as I and others have pointed out, is the limitations of their industrial capacity to make and supply higher-precision higher-strength parts needed to keep diesels running. I'm not assuming they've forgotten how to build diesels; I'm saying that, knowing how to build diesels, under conditions of a weaker industrial base, they WILL revert to steam -- an example of "appropriate technology". And who says it will die out as fast the second time around? It seems an accident of history that "advanced steam" tech wasn't developed that would've kept it competitive for longer... $\endgroup$ – Tristan Klassen Feb 21 at 16:24
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    $\begingroup$ @Tristan Klassen BTW the Minnesota transportation museum is restoring several early Diesel engines, but the steam engines are more sexy and get the donations. They use the small working Diesels for visitor train excursions because they can't get any of their steam engines working, and keeping up the Diesels is cheap and easy. $\endgroup$ – DWKraus Feb 21 at 16:24
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    $\begingroup$ The earliest steam engines were fairly simple devices. However, they used a lot of fuel for their power. Then they developed more efficient steam engines with multiple sets of pistons -- one set for high-pressure steam, and other sets for lower pressure after the steam had pushed the first set of pistons. They were more efficient, but they became extremely complicated. $\endgroup$ – NomadMaker Feb 21 at 22:02
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    $\begingroup$ High quality metals in post apoc situations are going to be insanely easy to find. Alloys don't just go poof. They may decay, but still. $\endgroup$ – Yakk Feb 22 at 5:34
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The apocalyptic event in question physically destroyed a large number of cities, and killed most of the human population.

If most of the humans are dead, the economics, the society and effort for rebuilding will be very different. One may even see a return to even more elementary technologies since oil extraction itself can be unfeasible, and coal anyway didn't exist in your world.

Thus, since most of the world is desolate of humans (and by extension other animals), if there is enough tree cover, then the trees could be used to create charcoal/be burnt as firewood directly to power steam engines.

This could help make steam locomotives simpler to bootstrap than oil/coal.

bonus question: with the majority of locomotives using oil rather than coal, how much faster would diesel locomotives have overtaken steam

The problems with oil in general are:

  1. Special Storage and transport requirements - oil spills, coal stacks
  2. Refining capabilities - most coal can be burnt as is, oil as used today is highly refined
  3. Extraction technology - coal is simply mind from surface / sub surface mines, oil can be offshore, or deep well, or shale oil etc

Thus, depending on how the oil is available in your world (good quality or not, easy availabiltiy - say a lake of oil was covered with nitrogen/ inert gas and hence is in tact, natural terrain allows for easier transport etc), you can tilt the benefit towards oil.

Are steam locomotives more viable than diesel in a post-apocalypse?

It therefore all depends on energy efficiency and resource abundance that society is able to achieve. Think of it in terms of how much 1 unit of fuel lets you move intended weight by, and how easy it is to come across that 1 unit of fuel. (The fuel is spent in transporting fuel weight + dead weight + intended weight).

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  • $\begingroup$ All largely correct until the last bit. There are any number of technical/economic reasons why things are often done in ways that aren't the most energy-efficient. And trains aren't planes or rockets -- hauling fuel weight isn't a big issue. $\endgroup$ – Tristan Klassen Feb 21 at 16:10
  • $\begingroup$ @TristanKlassen you are right about the fuel weight for oil/coal. I was keeping all fuels in mind where firewood efficiency can be ~5% as compared to ~35-40% for coal, and still higher for refined oil. $\endgroup$ – Anshul Goyal Feb 21 at 17:20
  • $\begingroup$ I do suspect that if steam was picked instead of Diesel, it would be due to the ready supply of wood (not coal) and not due to steam's superiority. $\endgroup$ – DWKraus Feb 21 at 17:25
  • $\begingroup$ @DWKraus You are right, corrected for abundance of resource impacting the usage. $\endgroup$ – Anshul Goyal Feb 21 at 17:31
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    $\begingroup$ The big resource for steam is water, by weight they consume far more water than fuel and as the OP stated these are large deserts. $\endgroup$ – John Feb 21 at 19:53
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A resounding "Yes."

Steam locomotives came about before Diesel for three reasons.

  1. They're easier to make. The thing most people don't realize is that diesel engines aren't just "a gas engine burning diesel." The most notable difference is that the diesel is combusted by subjecting it to high pressures. This means you need two things to make a diesel engine, both of which aren't feasible in an apocalypse:

    1. High-quality steel so that it doesn't warp or rupture from the pressure
    2. Much higher machining tolerances than a steamer, especially around the piston heads.
  2. They're more maintenanceable. Even if you can get an engine, diesels have a lot of little fiddly parts that can get broken, gunked up, or otherwise FUBAR'd. As @TristanKlassen mentioned, a real life example of this is Guatemala during the '60s.

  3. They don't expend non-renewables. This is a post-apocalypse wasteland we're talking about here. The only sources of diesel, automotive oil, and half a dozen other things you must have to run a diesel engine will be what you can scavenge. This is especially the case with the oil and diesel; even if you can find a large enough supply, it'll go stale before long.

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    $\begingroup$ True on the engineering, not on the fuels. Both Steam power and Diesel engines require fuel, but if using liquid fuels, they both can run on pretty much the same things, and these liquid fuels are rather easy (but expensive) to manufacture from crops or plant material. $\endgroup$ – PcMan Feb 21 at 15:05
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    $\begingroup$ I don't understand #3 - how do you think steam gets generated without expending non-renewables? $\endgroup$ – Aganju Feb 21 at 15:35
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    $\begingroup$ I wouldn't say it's that steam is easier to maintain, but that it's more maintainable if your supply chain is unreliable. Your point 1.2 explains why. A steam locomotive shop can make its own replacement parts, while a diesel locomotive shop mostly installs and services spare parts delivered from the factory. This is the usual explanation of why, for example, Guatemala in the late 1960s experienced a partial reversion to steam: IIRC, shifting exchange rates meant they had a shortage of imported spare pars for their diesels. $\endgroup$ – Tristan Klassen Feb 21 at 15:39
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    $\begingroup$ @John I meant "unreliable supply chain" on a different scale. Steam and diesel both need fuel to run. I meant, if your supply chain for building and repairing the locomotives is unreliable, steam wins because it's somewhat tolerant of small-scale, almost artisanal local production. $\endgroup$ – Tristan Klassen Feb 21 at 18:33
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    $\begingroup$ Yeah, I'd say most post-apoc scenarios would favor steam, but "lots of desert and little coal" doesn't look good for it... which makes me doubt their ability to get to a stage of building any railways... $\endgroup$ – Tristan Klassen Feb 22 at 3:36
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As they rebuild their industrial capacity, steam will come before diesel. Yes, steam requires a significant industrial base to build and maintain, but diesels require even more. So even if they were already at the diesel level before the apocalypse, they'll revert to steam afterward, for some time at least.

Fuels aren't the issue. Diesel, and even more so steam, is surprisingly flexible in what it can run on. Honestly, my concern is the "desert" part. In real life, railways in deserts were generally the first to dieselize. I suspect that's why some places (namely, inland Saudi Arabia) didn't get railways built at all until diesels were available. Steam needs lots of water.

So, they'll redevelop steam first. The question is whether they'll rebuild much in the way of railways at all until they redevelop diesels because of that water shortage!

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It depends on their technology.

Diesel is much more efficient and versatile, but require a much more sophisticated level of metallurgy and fabrication capability than steam does. Mind you, this is diesel-electric, which also requires the knowledge and tools to make high-power generators and electric motors.

how likely is it that this society is already able to refine oil to the extent needed to run diesel-electric locomotives?

The fuel is almost irrelevant, for either case.
Both can run on a variety of reasonably easily made fuels. The difficulty with making fuels have to do with efficiency, and economy, and sustainability. The actual processes are comparatively trivial.

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  • $\begingroup$ I suspect that if you have the metallurgical and machining know-how to make useful steam trains, then replicating the earliest diesel engines should be straightforward. $\endgroup$ – Starfish Prime Feb 21 at 13:41
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    $\begingroup$ @StarfishPrime Not even close. Diesel uses similar mechanical constructs, but much higher pressures and requires finer tolerances. Try 220 bar in a diesel cylinder during power stroke, vs 17 bar in a steam locomotive. Stroke duration of 15 milliseconds in diesel vs 120ms in a steam engine. Same concept, way different execution parameters, way better structures needed. $\endgroup$ – PcMan Feb 21 at 15:02
  • $\begingroup$ Right. It's the difference between theoretical knowledge and actually having the industrial base to implement it. $\endgroup$ – Tristan Klassen Feb 21 at 16:33
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Unless your apocalypse took place in a location with antiquated rail technology (like most of the US) that you're re-using, neither steam nor diesel is a good solution. Your (exitsing, rebuilt) locomotives are electric. You can run them with power generated by hydroelectric dams, which should be easier to restore than building an oil refining or wood/coal supply chain. See for instance the railways in Europe.

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  • $\begingroup$ OP does say "diesel-electric locomotives became the dominant form of rail transport." $\endgroup$ – Tristan Klassen Feb 22 at 23:31
  • $\begingroup$ @Tristan Klassen: Yes, but the OP didn't say that they STAYED the dominant form :-) Unless the apocalypse happened when the local society is still in that transition point, you're likely to have a lot more salvagable/restorable electrics. Just as the choice between steam & diesel depends on the time. Early on, you have lots of steam locomotives. Later, you find them only in museums & tourist rides. $\endgroup$ – jamesqf Feb 23 at 17:30
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You forgot one thing: steam engines can burn wood.

Wood-burning is a very inefficient method of energy production, but in a post-apocalyptic period, how much trade and travel is there actually going to be? Remember, until sometime in the early 1900s, if I remember correctly the average person traveled less than 350 miles from his home YEARLY.

So efficient, long-distance personal travel is immaterial and as for goods, wagons and boats would suffice, as most of the population of the world lives near some type of navigable seaway.

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Assuming that this world has lost the manufacturing facilities to make diesel parts, or even the ability to refine proper oil to lubricate the engines, and all diesel locomotives that still exist no longer run, a steam locomotive can be operated in a low tech environment, as it can run on a lot of fuels (including oil), and the parts and lubricants needed to run the steam locomotive are much simpler.

However, you would have to build the missing infrastructure to support a steam locomotive, that has long since been abandoned.

Steam locomotives can run maybe 80-100 miles before they have exhausted their water supply. So you would need large amounts of water at regular intervals, plus a way to get that water into the locomotive. This is the large barrel on stilts that you might see in some old west films, adjacent to the tracks.

You would also need a lot of maintenance facilities to be built - steam locomotives require more frequent rebuilds, due partially to the running parts being exposed to a very corrosive substance: superheated live steam. That is why most cities in the steam locomotive age, even smaller ones, had at least one roundhouse - to rebuild locomotives.

And finally, you would have to rediscover the art of building and running a steam locomotive. Compared to a diesel, they require a lot more attention while running, to insure the water level doesn't get low (boiler explodes), to keep the running gear lubricated (largely a manual process), and manage the firebox to get an even distribution of heat, to mention a few.

The primary reason diesel locomotives replaced steam locomotives in the 1950's and 1960's was the lower maintenance costs. Diesel locomotives had a much greater range, didn't have to stop for water, didn't need to be rebuilt nearly as often, and made more efficient use of fuel than steam locomotives.

But, if you don't have the infrastructure to keep a diesel locomotive operating, then the steam engine can be operated with a lower level of available technology.

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The hot-bulb engine will run on most fuel. They use a compression ratio of 3:1 to 5:1, a diesel uses around 15:1 to 20:1. This lower pressure allows it be made from lower-grade steel. However, they are about ⅓ of the efficiency of modern diesels, and require a blow-torch to heat it to running temperature before it will start.

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  • $\begingroup$ Yep. at which point you are better off using the same liquid fuel to heat the boiler on a decent steam locomotive. The advantage of diesel over steam is fuel efficiency, and somewhat power per volume. $\endgroup$ – PcMan Feb 21 at 19:40
  • $\begingroup$ The Flying Scotsman ran at 17atm, and was very complex. My point is that hot-bulb engines work well at lower pressures. If your metallurgy is not very good the hot-bulbs are easier to built $\endgroup$ – CSM Feb 21 at 21:46
  • $\begingroup$ you just prove my point. The hot bulb engine is about 12% efficient(vs steam locomotive at 15%, and diesel engine at 35% for early designs). And a hot-bulb motor still runs at much higher compression than a Steam engine. so you have both harder to build, and less efficient. They are much more tolerant to lowgrade fuels though. The compression ratio is only 14bar, but at power stroke it reaches 60 bar, same way a diesel is only compress ratio 20, yet needs to survive 240 bar $\endgroup$ – PcMan Feb 21 at 22:20
  • $\begingroup$ You have to maintain the track to arguably a higher standard with steam (hammer blow) than diesel. Motive power is only part of the total railway system. $\endgroup$ – Michael Harvey Feb 21 at 22:41
  • $\begingroup$ @PcMan A good "traditional" steam locomotive was only 6-7% efficient. 12% is entirely technically attainable, but the realm of Chapelon, Porta, Wardale and beyond. I will argue that such "advanced steam" is a viable technological path for redeveloping industrial civilization... $\endgroup$ – Tristan Klassen Feb 22 at 23:28

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