While Harrison has his own Lionel layout, he has really taken an interest in the O scale layout I am building. As such, I try and think of projects we can work on together. So far, he has helped me squeeze closed "suitcase" connectors during my wiring projects; he has squeezed the trigger on my cordless drill (while I hold it steady) to screw in wiring clamps; and he is always willing to test track clearances and such.
After all the "work" is done, he likes to test out the track by rolling some of his trains back and forth. He has to stand on a stool to reach, but he is learning to be careful and balance properly.
Sarah and I were discussing recently whether his interest in trains will last throughout his life or wane as he gets older. I have no idea, but as she put it: "You really do a lot of train stuff with him". It is true. I don't mind if he has other hobbies in the future, but I will always try to include him in mine if he wants.