My question is pretty simple, if the steam locomotive were invented during the middle to late medieval period, what would it look like? We are just going to ignore how it was invented.

My guess is that it would be fairly small and light. I’d also guess that the boiler would have wood lagging (insulation) around the boiler (mainly) and that the cylinders are in the outside and not inside the frame. In short, like the early steam locos of the 1900s century.

The metal working of the period would also limit the boiler size, though it would probably improve quite fast ... but let’s just ignore that for now.

If they just couldn’t do it, then let’s give them the minimum needed to get a functioning steam locomotive that could pull a (short and light) train.

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    $\begingroup$ Why would it look any different from a 1802 steam locomotive. If we are ignoring how it was invented, then they should be the same. $\endgroup$
    – kingledion
    Sep 18 '18 at 16:57

You can basically follow the history: https://en.wikipedia.org/wiki/Steam_engine#History

Lower quality of metal means that steam engine will be bulkier, or have less power. The biggest issue is containing the steam pressure, and weaker metal means you need more of it. So you get a loco that can barely pull its own weight, with very little power left to pull carriages.

In fact, in reality steam engines started out as being too heavy to move themselves, and were used to pump water out of mines, or later to turn machinery in factories. So by the time you invent a locomotive, you should have factories.

Also, another limitation of medieval technology is precision of manufacturing. The piston has to fit the cylinder just right, the valves directing steam to different sections of the piston must be air-tight and open at just the right moment. It would take years of trial and error to build one, and errors could easily kill the craftsmen and destroy the workshop.

Compared to that, horses are much easier. You can have a railroad with horse-drawn carriages. More privacy and independence this way too.

  • $\begingroup$ Well the steam distribution and timing isn’t to hard to pull of, just look at the earliest steam locos and engines. But yeah, you’re probably right about most of it. $\endgroup$ Sep 18 '18 at 16:39
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    $\begingroup$ Agreed. Without industrial-revolution-level technology you're going to be limited to an atmospheric steam engine, which is even heavier (for a given output) than a pressurised engine. I'd say that your most likely 'locomotive' would be a stationary engine, winching the train along. $\endgroup$ Sep 18 '18 at 17:21
  • $\begingroup$ +1 and agree with @MattBowyer that only feasible option is immobile engine using cable to winch carriages. Also note that metal rails are not feasible prior to improvements in steel manufacture, which ironically require immobile steam engines to blast air to blast furnaces! Wooden rails would not support an inefficient heavy locomotive. It really is not feasible to be railroading until all of the technological elements come together in the Industrial Revolution. $\endgroup$ Sep 26 '18 at 12:52

your locomotive might look like the Fardier, built by Nicolas Cugnot in the XVIIth century in France:

By unknown/F. A. Brockhaus - Brockhaus Kleines Konversations-Lexikon (5. Auflage 1911) zeno.org, Public Domain, https://commons.wikimedia.org/w/index.php?curid=3170175 enter image description here


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