Something I found very interesting with the release of Update... was it 4 that brought T2 trains? I don't know, but it's the T2 steam train.
On the T1, the engine is on the body, driving a crankshaft on the bottom of the locomotive, which probably driveshafts its way to the trucks to actually drive the locomotive. That arrangement makes sense to me, as it turns the locomotive into, essentially, a steam-powered diesel-mechanical locomotive: The "engine" section is separate from the "running gear" section. Though I will note they're clocked wrong, all 180-degree opposed. Real engines are set at 90 degrees so that when one cylinder is at one extreme end (and offering precisely zero power), the other is in the middle of its power stroke.
On the T2... it moves to standard steam locomotive design: The running gear is mounted with the wheels, so the wheels themselves serve as the "flywheel" of the engine. The problem? It's articulated! As a matter of fact, it's dual-articulated... and real-world articulated locomotives had to go through extensive engineering to convince the steam to flow down into the articulated drivers and back up to the exhaust stack without pouring out the sides of the locomotive. A dual-articulated locomotive like these... just doesn't make sense, from an engineering standpoint. Mechanical power I could see transmitting to a rotating truck via a driveshaft; T1 does that. Electrical power, easy, just a cable. But steam? Heavy piping and lots of extra joints in order to make that work. And to do it twice on one locomotive...
... actually makes sense, given the sharpness of the curves present in CoI. Why? A standard, single-articulated locomotive like the Union Pacific's Big Boy (only one I'm casually aware of), and many others around the world, would stick way off the outside of the corner and require an extra track clearance of several squares for sharper corners, nevermind such an overhang easily overbalancing the poor locomotive, especially at speed, and causing it to derail extremely easy. And that's before you even consider the structural difficulties of a heavy, 15-meter water-filled boiler only being supported on one end because the articulated front of the locomotive is too busy following the tracks to be under it.
Solution? Double-articulation! Probably even higher maintenance than single-articulation, but if it gets the job done... Pretty sure real-world solutions were either to build straighter tracks or couple multiple, shorter locomotives together, rather than making such a complex single locomotive.