Chronic Logic

Since CL's got a way to store the last step, then all they need to do is make a dropout stack of that data structure.  It's very easy.  Just have a static array, and pretend it's a ring, then keep a pointer to the most recent step.  As you undo/redo, you can move the pointer backwards and forwards.  You also keep a pointer to the tail so you know when you've gotten to the bottom of possible undoing.  This is something they teach every sophomore CS student.  (like me last year.)  :P

Certainly, the train shouldn't be able to drive through steel.  This isn't quite as serious as detecting all possible steel collisions, but would prevent people from building things through the road (which just looks silly).