And they operated into the 1970's.
London is going to be a hard nut to crack, ti's already got several levels of tunnels under the city.
There are some real issues with underground tunnels, especially ones to small to be traversed by people. People are "universal power tools" and can get in there and fix unusual problems. If a rail car the size of a trash can gets stuck in a tunnel you can send a man down.. figuruing out how to get it out is going to be a real trial.