Dead tree (aka wood) is one of the oldest and hardiest building products known to man (aka Dads). Trains and train tracks are fascinating for many kids aged ~3-8. Ergo, wooden train sets are an addictive and hard wearing addition to any toy collection. A box of track, trains, and stations etc, will keep your kiddos busy for hours, and be of huge interest to their visiting friends.
The problem with much of the wooden train infrastructure out there though is that it costs....a lot. Apparently these days there are outlets for bulk track (and more), but these options were not around when I was considering how to extend our starter set. Fortunately, there were some DIY websites with information on how to make track with a router. Once started it became a bit of a perpetual project to create bridges, long track, stations, shipping, tunnels, houses, ports and even ferries. The end result is the ability to make a layout that covers entire rooms. Which is just what the kiddos want. Even those that have outgrown the "train phase" still have fun helping to set up layouts for their younger siblings.
I guess it's cheating, but tallying up the individual components counts towards the Dad50 list (if you disagree, re-read the rules), giving #26 to 33 in this post.
Bridge supports. Twice the height of normal elevated track.
Ferry and container ship port
House for a favorite character. Complete with parking. And a bathroom.
Having previously been a "train-fixated" kid myself, and after watching my crew closely, I've tried to put a finger on what the attraction is. I think this sums up what's going on upstairs when your kid spends countless hours absorbed in train play:
-unplug (from the TV) and play