The Oregon Zoo in Portland has a two foot gauge railroad that goes around the zoo and out through Washington Park
Unlike some others the steam locomotive runs on steam. Zoo admission is required to ride the train. There is a station in the Zoo and in Washington Park.
Washington Park's railroad is really pretty spectacular. They've got some interesting streamlined consists, a number of the old traditional V-4 powered engines in various styles (including the model of the Aerotrain in the opening post, which has to be rare as hen's teeth), and a pretty good length run. It's also a beautiful park in the summer.
The Aerotrain was built by GM as they were a sponsor (along with some of the local railroads) in 1958 when the new zoo opened. I seem to recall that SP crews laid out and built the line to Washington Park. The "Zooliner" is always my favorite to ride on when I've been there. Peter