Cincinnati is the site of the country's largest abandoned subway tunnel. But "abandoned" is not quite the word, as construction slowed to a stop in 1925 before even half of the 16 mile line was completed.
Welcome Aboard the Fort Collins Municipal Railway, featuring Birney Safety Car #21, restored and running on dedicated right-of-way. Also links to trolley museums, other "heritage trolley" sites and general rail info.
"Deep down beneath the choked streets of London was a railway which once ran busily for nineteen hours a day. It carried no passengers and its trains had no drivers or guards. Yet this seemingly strange system was one of the most successful railways in the world."
You read that right. This is a map that displays particular smells (horrific and sublime) that can be found throughout the NY?C subway system. Very funny and kinda useful.
The American Public Transportation Association has a lot of great statistical data about all the transit systems in the U.S. Very useful reference for comparative research or just browsing.