Somewhere, I got into a discussion with somebody about how many cylinders does a steam locomotive have. The other person said only two, but I wondered. A common configuration for a steamship is three cylinders of decreasing size that use the steam three times.

What I found out about steam locomotives, is that the usual configuration for a locomotive with one set of drive wheels (like a 4-8-4) is two cylinders, but the those pistons are double-acting or two-stroke, with a power stroke in each direction. Steam is injected alternately into the front and the back of the cylinder. While the cylinder is exhausting steam in one direction, it is being injected with steam in the other.

An articulated locomotive (e.g., a 4-8-8-4 like the Big Boy) is like two locomotives with a single boiler and two sets of driving wheels - and two cylinders driving each set of drivers. The drivers swivel to allow the locomotive to go around curves.

I've heard of other configurations, notably locomotives that have a third cylinder in the middle underneath the boiler, but that sort of configuration is rare.

In the U.S., Shay Locomotives were used for logging. They had vertical cylinders on only one side of the locomotive, and those cylinders moved a crankshaft that powered the driver wheels through gears - producing lots of torque at very low speed to the locomotives could handle steeper grades. Shays originally had two cylinders, but later Shays had three - these are absolutely fascinating locomotives. They don't fall in the "beautiful" class, but they sure are fun to see in action. I saw an operating Shay at a tourist railroad south of Yosemite in the Spring of 2019. I took my kids for a ride on that railroad way back in about 1978.

The "steam locomotive" entry in Wikipedia is fascinating, and will no doubt become even more interesting as time goes on:

https://en.wikipedia.org/wiki/Steam_locomotive

-Joe-