We live fairly close to the museum (about 45 min away) so decided a few months ago that it would be a nice little day trip. I have to say that was one of the strangest places we've been to. We got there in the morning on a Saturday, but not right when they first opened, and there was NO ONE there. The only people we saw were a few employees wandering around setting up signs for a big event the next weekend. The museum is layed out on a few blocks that used to be an actual town so there are regular streets you walk around and then there are some trains outside and some on display in buildings. We had no idea where we were supposed to go. We wandered around sort of aimlessly. We did finally find some trains to look at but it was kind of weird. Since no one else was around it almost felt like we were going into places that we weren't supposed to be. There also seemed to be a lot of old rusted trains around. After awhile we saw the big train come back to the station with some people on it so we thought maybe everyone took the train ride. But after the train came back we still only saw one other family and they looked like they didn't know where to go either. It was a case where we wished there was a bit of a crowd so we would know where to go.
Maybe someone else had a better experience there but we didn't like it. At least it was free. More recently we've been to Travel Town in Griffith Park which is in LA and it was a similar type of Museum with big, old trains on display but we liked it MUCH better. It was more crowded but the trains and the whole area was nicer. They have picnic tables - lots of Thomas birthday parties going on. They don't have a big train you can ride but they do have a little train you can ride around on. Also, I believe there is another area of the Griffith Park that has a train you can ride and if I'm not mistaken, I think it used to be Walt Disney's train (i thought i heard that one time but i'm not positive).
There is also a model train museum in San Diego's Balboa Park but we've never been there so I don't know much about it.
My suggestion would be to go to the museum in LA and try to get there early so you can find a parking space.
