Here is an idea, if you have some old trains that are in poor cosmetic shape, you might try repainting them in a children’s theme for your kids or grand kids. A friend of mine painted up 2 sets of old Marx trains for his grand kids, one as a Batman Train and one as a Princess Train. It’s just a simple spray paint job with stickers made on the computer and printed out.
Warning: I would only paint them up like this if the cars and engines are in bad shape, there is no sense in painting up a nice looking piece of history.
Here is a video of the result. I think they worked out well and the kids love them.