Locomotives have a certain sort of elegance and power - elegance and power which is captured through art of trains, and even toy train art captures some of these elements. The style doesn't take away from the mental imagery inspired by the picture, and for the room of a young child, the style of any photograph or poster will do well to both capture and kindle the imagination.
When looking around to pick up toy train art, look around at the room you plan on decorating. Does the scheme otherwise look plain? Then realistically designed trains might be better to hang up on the walls, providing a bit of flash to an otherwise blank canvas. If there are other posters of cartoon art on the wall, then a design closer to cartoonish might suit better.
It's important to think about the mind of the young child when making a decision on train art. If they like trains, are they old enough to prefer a more real style, or still young that collage train art and cartoon train art will catch their eye and their attention?
Toy train art can also come with train like phrases and words on the pictures, like yield or stop, and could be good in exposing a young mind to a beginning vocabulary. Remember to consider what other posters and pictures surround the new addition you're placing on the wall - a realistic train might look at home next to a wall of flowers, but collage train art will sit better with similar cartoonish styles.