Fascinating article from the BBC on the Post Office Railway (otherwise known as mail rail).  This driverless narrow-gauge line carried mail for over six miles from sorting office to sorting office under the streets of London from 1927 to 2003.

That fount of all knowledge Wikipedia (and it’s sometimes even right, too) tells us that:

The main line has a single 9 ft diameter tube with two tracks. Just before stations, tunnels diverge into two single-track 7 ft diameter tunnels leading to two parallel 25 ft diameter station tunnels. The main tube is at a depth of around 70 ft. Stations are at a much shallower depth, with a 1-in-20 gradient into the stations. The gradients assist in slowing the trains when approaching stations, and accelerating them away. There is also less distance to lift mail from the stations to the surface. At Oxford Circus the tunnel runs close to the Bakerloo line tunnel of the London Underground.

Not sure about the gauge though, and its potential for modelling is perhaps low…

And now there are now plans to reopen this fascinating little line as a tourist ride.  And it may be as quick as CrossRail to get from Paddington to Liverpool Street, if rather less comfortable.

Read more on the BBC website.