Here's an underground experience you won't find on the tube map. Built in 1927 to beat traffic congestion, the Post Office Railway was a subterranean train line used to move four million pieces of mail beneath the city streets every day until it was shuttered in 2003. Revamped and opened to the public for the first time in 2017, Mail Rail now delivers delighted visitors around the once little-known tracks below the largest sorting office, in trains based on the original designs.
Mail Rail is the headline attraction, but it's worth popping across the street to the surprisingly fascinating Postal Museum, which traces the history of Britain's postal system and how complicated it was to set up.
Claustrophobes beware: Mail Rail was originally made to carry post and not people, so while the carriages are wide enough for a single person, staff are sometimes intent on squeezing two people into one seat.