My son is three. Will turn four this summer. He wasn't interested in building the table. Here are the things he was interested in (and my appreciation for people on this forum for clueing me in on many of them).
1. Lights. I have a bunch of accessories that light up and/or flash. Son loves to "night run" the trains, which involves having all the accessories on, and then turning off the lights in the workshop.
2. A loading dock and plenty of gondolas and hoppers in the train's consist. Other than the engine/tender, and caboose, the consist of my son's LC Pennsy Flyer is comprised of gondolas, hoppers, and ore cars. A couple were purchased from K-Line or Lionel complete with LCL's or crates, but I then went to Hobby Lobby and purchased items that - while much larger than scale - would work with his toddler fingers and that he could load/unload with ease. Things like these miniature sacks (which in scale are something like 13 feet long). Honestly, most of the time is my son loading a gondola, doing one lap on the 4x8 layout, stopping it, unloading, and repeating.
3. Vehicles. When the train is doing it's lap around the track, my son is playing with the bus or trucks on the layout. I know he's waiting for the day I'm not watching and he can see if the truck or train would win in a collision, but so far it hasn't happened. If there was anything I wish I had done, it would be perhaps to expand the layout table a bit to allow trucks to run outside of the track. Right now space is tight, but I'll probably do this sooner rather than later, because it's something he really likes to do.
4. Lionchief (Lionchief Plus). Kids are able to figure this out pretty easily. I have three LC engines, and one LC+ engine. I have conventional as well, but they don't see much run time because the LC/LC+ system is so easy to control.
