Seventy feet under London, little red trains once raced through the dark with nobody on board. For 76 years, London's Post Office Railway, later called Mail Rail, carried up to about four million letters a day beneath streets full of people who never knew it was there. This animated explainer follows the driverless mail train from the traffic jams that inspired it to the museum ride you can take today.

Chapters:

0:00 Intro

0:25 The city that jammed: why go underground?

1:33 Tunnels at war: 1915 to 1927

2:30 A railway for letters: 10.5 km, 8 stations, 70 feet deep

3:31 No driver needed: how the trains ran

4:53 From sack to train: the stations

5:52 Around the clock: millions of letters

6:56 The last train: why it closed in 2003

7:49 Sleeping tunnels: 2003 to 2017

8:43 All aboard: the ride today

• How a third rail, track sections and switch cabins let trains run with no driver

• The 1860s air-powered mail tunnel, and the hidden art stored in the empty tunnels during the First World War

• Paddington to Whitechapel: two-foot track, tiny tunnels and up to about 40 mph between stations

• Chutes, lifts, conveyors and bins: how mailbags got from the sorting office to the train

• Why it closed in 2003, the fourteen quiet years, and the Postal Museum ride that opened in 2017

• English AI voiceover, original synthesized music and SFX, mixed to -14 LUFS

• Original vector animation, 1080p 60 fps, fact-checked, no real brand logos

A hidden machine that ran beneath one of the world's busiest cities for three quarters of a century.