28 January 2014 Last updated at 00:03 GMT
The Post Office Railway, otherwise known as Mail Rail, is a 6.5 mile (10.5 km) train system deep under London's streets.
Once used to shuttle post between sorting offices, the tunnels and mainly driverless trains have been out of service for more than 10 years.
But the network has been maintained throughout this time, and now the British Postal Museum & Archive is planning to open part of the network to the public.
BBC News went for an exclusive peek around.
Video Journalist: Dan Curtis; Camera: Luke Finn"
To see the BBC video click the link under:
BBC News - Mail Rail: The hidden trains under London's streets: