[5] In 1911, a plan evolved to build an underground railway 6+1⁄2 miles (10.5 km) long[6] from Paddington to Whitechapel serving the main sorting offices along the route; road traffic congestion was causing unacceptable delays.

Most of the line was constructed using the Greathead shield system, with limited amounts of hand-mining for connecting tunnels at stations.

The Communication Workers Union claimed the actual figure was closer to three times more expensive but argued that this was the result of a deliberate policy of running the railway down and using it at only one-third of its capacity.

[4][14] In April 2011, an urban exploration group called the "Consolidation Crew" published accounts of illicit access to the tunnels.

The study is to establish how the original cast-iron lining sections, which are similar to those used for many miles of railway under London, resist possible deformation and soil movement caused by the new works.

Digital cameras, fibre optic deformation sensors, laser scanners and other low-cost instruments, reporting in real time, have been installed in the vacated tunnel.

[18] In October 2013, the British Postal Museum & Archive announced that it intended opening part of the network to the public.

It was planned to open a circular route, running beneath the depot at Mount Pleasant with a journey time of around 15 minutes, by mid-2017.

[25] In its first year of operation (2017–2018), the trains performed 9,000 trips totalling 6,213 miles (10,000 km), with the railway and museum hosting over 198,000 visitors.

In 1910, a 450-metre (1,480 ft) tunnel railway opened in Munich, Germany between München Hauptbahnhof and the nearby Post office.

[32] The Chicago Tunnel Company, in operation between 1906 and 1959, delivered freight, parcels, and coal, and disposed of ash and excavation debris.