The Post Office Underground Railway once used below London is set to open soon to the public:
The Post Office Underground Railway—AKA the Mail Rail—was the world’s first driverless electric railway. It launched in 1927 and was used to transport tons of post from one side of London to another, with stops at large railway hubs such as Liverpool Street and Paddington Station, where post could be collected and offloaded for transportation around the rest of the country…
The idea is to create special battery-powered passenger carriages to take people from the car depot and some of the tunnels in a one-kilometer loop. Visitors will be taken 70-feet underground, through Mount Pleasant Station, and will stop to view audiovisual displays recounting the history of the network and what it was like to work down there…
The railways have a 61cm gauge (the width of the track), on top of which small carriages traveled without drivers thanks to electric live rails. In the stations there are two tracks, with carriages going in each direction.
The service continued to operate until 2003, when it was closed down—it had become much cheaper to transport mail by road.
Looks like a cross between a Disney ride and the Tube. I suspect there may just be a tourist market for this ride…see this post from April 2011 about people exploring the system as well as the interest in looking at underground Paris.
Such lines underneath cities may not be all that unusual. Chicago had an underground delivery system as well since it was far more efficient to move some items underground away from the street-level traffic. This was also the impetus behind creating Lower Wacker Drive. How many major cities have such tunnels underground, how many of them are well secured (free from ne’er-do-wells or flooding issues), and are these all tourist opportunities waiting to be opened?