At our house, my son now 7, loves pushing around the cars. For the last few years I have kept a siding to set the locomotive on, so he can still push the cars. We have two rules, do not push the locomotive, and do not pick up the cars. We always have another siding to so he can rearrange cars. He has been good about these rules since he was a little over three. In the following years, he has learned to operate the transformer and last year he was running two trains via TMCC all on his own.
I have a old set from my dad that we use under our tree (since 1970 & before I was born), and a couple of years ago I purchased a legacy RTR Conrail set that we have also used under the tree. I started with tubular track, then switched to Fastrack. The Fastrack is very easy to use, but expensive and noisy, and will still pull apart at times during normal use. The original 60 watt transformer from my das's set still works good, and have no problems the CW-80 that I received with my Conrail set.
We have since expanded and are now building a bigger layout in our basement, my track preference has changed, but I don't have any regrets on the purchases that I have made to get me to this point.
Whatever you decide, I'm sure you and your son will create some great memories. I have had fun with our evolution in our train sets and have had fun watching him on the basement stairs running our trains without him knowing I was there watching.
