"What I want is to have a train that will move in a straight line, then arrive at its path_rotate_corner and turn 90 degrees, continue down a straight line, rotate another 90 degrees, etc, etc., eventually forming a square path. I can't figure it out, though. Any help would be really appreciated."

- David Kelvin

...This sounds like THE typical use for a rotating train ;) Have you made sure that the ROTATION bit has been set and that the "rotate" field for the entity has been set to "99 99 99", where 99 99 99 are the speed of rotation in the X, Y, Z planes ( 0 for no rotation in that plane). You may, instead, want to use the ANGLES/"angles" combination so that the train spins around exactly 90 degrees. Rotating trains take a while to gain control over, so don't get too frustrated and keep your examples small and simple when you start.

- Levelord