When my 14 year old son was three or four he got the first parts to a wooden train set. We bought a Sesame Street set because it was relatively inexpensive, and a couple of "Thomas the Tank Engine" pieces because that's what he wanted. Over the years, until he was seven or eight, adding to that set was a regular source of rewards, Christmas presents and birthday gifts. My guess is that by the time we were done, we had almost $500.00 in trains, tracks and other pieces. I will say though that they were played with, a lot. When we were cleaning up two years ago getting ready for the baby, the trains went in the attic.
Friday night I took the kids to the bookstore, and they had a train table, that the baby loved. She hated to leave it. Saturday, I sent my 14 year old up in the attic for the train set. Today we have been playing choo choo.