There is a Twin City Model Railroad Museum right here in St. Paul, MN. I used to go there with my son quite frequently when he was little—that would be 15-20 years ago!—but I haven't been back there for a long time. I used to think it was pretty impressive—until I saw the video of the one in Hamburg.

There ought to be a directory of all the public layouts in the world, but I wouldn't know where to find such a thing. Every major city must have one worth visiting.