Mail Rail is the former underground railway used by the Post Office. It was in operation from 1927 to 2003, and covered 6.5 miles. In 2017 it was reopened as a visitor attraction by the Postal Museum and Islington Council. Visitors can take a 15 minute journey along the tunnels and former platforms, and then see an exhibition. The train seats are narrow, but otherwise it's a lot of fun!
I know you will really enjoy it!