GUI implementation with Racket
Broadly speaking, I'd say it looks pretty good. I'm a bit confused by the '1055'; I'm guessing that you're trying to wait until the train leaves the screen entirely? Perhaps you could write this as

(+ WIDTH (/ (image-width LOCOMOTIVE) 2))


... and also replace the 800 with WIDTH. Alternatively, you could return either 0 or 

(- (/ (image-width LOCOMOTIVE) 2))


when the train goes off the right edge, so that it makes its entrance smoothly.