Needed something to do in Balboa Park with a train-obsessed three-year-old. This fits the bill perfectly, and the prices are pretty reasonable.
The museum is in the basement of one of the buildings. There's a pretty big HO layout with a lot of accessories and working parts, some viewing stands for younger (and shorter!) children. There's also a couple smaller O-scale layouts, a few wooden train play rooms, and the biggest (and only) museum N scale work I've seen.
Add in a room for railroad clubs to set up a temporary layout, and you have a pretty good few hours of wandering around.