The miniature underground Royal Mail train that ran from Paddington to Whitechapel was arguably the most successful line in the world and ran for 75 years. It finally closed in 2003 and is featured in the film 'Hudson Hawk' - well worth hunting down. The following video was made when it was still in use and before the service was computerised.

The original idea for an underground mail train was first put forward in 1855 by Rowland Hill. The original proposal was that it should run from the Post Office at St.Martins-le-Grand to Little Queen Street in Holborn. Thomas Rammell came up with a scheme by which a stationery steam engine would drive a large fan which could suck air out of an air tight tube and draw the vehicle towards it or blow air to push them away - rather like a huge vacuum cleaner.

The principle was later adapted for use as a message handling system in large department stores and was known as the 'Lansome Tube' but implementing the system on a large enough scale to shift trains was deemed too expensive and the idea scrapped.

A test track was set up at Battersea and an early system built which ran from the North Western District Office to Euston Station run by the Pneumatic Dispatch Company but this proved unprofitable and was abandoned. However in 1911, work began on the 6 1/2 mile track from Paddington to Whitechapel.

More info at: http://www.mailrail.co.uk/