If you didn't want to go high tech, you probably could do it mechanically with hinges, bars, and plates. You could manually put any right, left, up and down limits. First go at it might be challenging, so maybe going the more expensive easy route of an electronic one would be better.
Originally, I was thinking of a simply gun through a hole in the wall. However, I don't own a tunnel, so I don't know whether that's an option or not. I do like the idea of kids squirting the cars. I bet a few dads would get some trigger time as well!