Page "London Post Office Railway" Paragraph 3

In 1911 a plan evolved to build an underground railway 6½ miles long from Paddington to Whitechapel serving the main sorting offices along the route ; even then, traffic congestion was causing unacceptable delays.

The contract to build the tunnels was won by John Mowlem and Co. Construction of the tunnels started in February 1915 from a series of shafts.

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