random PSA for people making steam trains in games or animations that aren't about trains (or even more importantly, those that are).
If you don't know how valve gear works, and your game has a stylized art style, please don't try to model the valve gear of your steam engine. I have a better solution for you.
A steam engine is conceptually pretty simple: you raise steam in a boiler, and you have cylinders on the loco. When you put steam in one side it pushes the piston back, and when you put it in the other it pushes the piston forward. Then you convert that linear motion to rotation with a crank on the wheel.
Valve gear is the rods and linkages on the side of the locomotive that sets the timing for when you put steam into one side of the cylinder or the other. It looks like this:
This is a daunting mess, and though it is possible to do it right, or to do it right enough that even a foamer won't complain (or, well, maybe they'll complain, but at least they'll know they're being petty at that point), you really need to dedicate some time to researching and understanding what it does. They're so complicated because that timing needs to change to put the engine into reverse, or to adjust the efficiency versus power of the the engine. There's all sorts of valve gear, the one in the picture above is "Walschearts valve gear"
But like. You don't even have to worry about it?
This loco uses stephenson valve gear. All of the actual valve stuff is happening inside the frames, hidden safely away where you don't even have to worry about it! Most early steam locomotives and a decent few later ones used inside-stephenson valve gear. The rods you see on the outside of the loco in this photo aren't part of the valve gear, they're just for moving the wheels.
YOU CAN HIDE THE VALVE GEAR!
All you need is three moving parts (and the wheels)
The crosshead and piston rod, which move back and forth linearly in the (static) crosshead guide and the cylinder. The full extent of their range, and the length of the cylinder, is equal to twice the length of the crank on the wheel.
The main rod, which connects the main drive wheel and the crosshead. One end moves in circular motion, the other moves back and forth linearly.
The connecting rod, which connects all drive wheels on the engine together, and moves in circular motion.
Now if you're making a realistic game or animation and your reference loco does use external valve gear, I would either choose a different reference or bite the bullet and learn how to do it right. But I see valve gear crimes in stylized media a lot more often.
Want to use a bigger steam engine that's a bit too new for Stephenson, but don't want to confront outside motion?
Here's two Pennsylvania Railroad designs
And now here are those same designs with two different flavors of poppet valve gear
