Train Collisions

Trains now have reduced friction and will bounce off each other when they collide!

This was initially to give trains a more realistic feel, only at lower speed, but I like how goofy it looks at high speeds. I will also slow down trains to give them a more realistic feel in the future.

Add an element of realism, allowing trains to collide with each other and bounce perfectly elastically. By default, they will not take damage below a certain speed, although that is configurable in settings.

Should be stable and not cause crashes. For now, automatic trains are unaffected by collision physics, to avoid crashing the game. Still some weird behavior at high speeds.