Just like last year, the Washington State History Museum in Tacoma has a model railroad festival for the holidays. It consists of 8 or so displays in various scales. It's surprisingly not crowded, and if I ever have concerns about Theo's attention span, they are assuaged by his ability to watch a single display (albeit from multiple angles) for over half an hour.
We finished about 3 hours later in the lego train room, where the kids could build their own trains. There was also a lego display, with a motorized tram which despite a lot of internet research, I have been unable to find -- it appears to be a custom modification of discountinued Lego City Public Transport Set #8404.
Check out all the photos. Afterwards we went to a nearby brewpub with an excellent Sunday deal: kids eat free and $3 pints. Best of all, the kids didn't fall asleep in the car on the way home, and so still managed to get their proper afternoon naps! The entire experience was so great we're going to do it again this Sunday.