Scrape with quick very light motions using an #1 handle X-acto knive with the straight blade (I forget the blade number). Make sure you don't put ANY pressure on the model you're removing the lettering from, just let the handle lay on your hand and let as much of the flat surface of the blade contact the lettering as possible.
This is a method I've used many times to remove numbers off freight cars so that I could renumber them. I'm sure it'd work on a locomotive as well, although I've never tried it, as I always do my own locomotives starting with an undecorated shell.