Ron,
I was given an airbrush back in 1981 and could not get the hang of it. I am just a spray can guy. I think if I were painting railcars like you I would have to learn how to use an airbrush. Spray paint out of a can is a little too thick for painting trains. As far as going over numbers, I think you can sand them off with fine paper. For light colors prime with white or gray and you should be okay.