What? That first part you got right, the rest, kind of bs. Only a few steam engines have gears (like the Shay, which neither of these are), and the valving runs the same either forwards or back. You do know that most steam engines are double acting stroke, right? unlike gas/diesel engines (single acting stroke), and the fuel (coal/oil) burns in a firebox that heats water to make steam (ergo, steam engine).

As for diesel engines, they’re not really diesel only. The diesel engine turns a big assed generator that powers electric motors that actually turn the wheels (you get more torque from electric motors than diesels, though not as much as hydraulics, but shit gets complicated with hydraulics).

And as for running an engine backwards into the mountain, those are probably pusher helpers. You can have them in the front, or the back, or both.