In message
,
Tristán White writes

Oooh, earliest reference I've found in Hansard of "tube railways" being
mentioned as an expression together and then referred subsequently as
"tubes" is from 7 March 1901

The term "tube" was used to refer to (bored) underground railways in
1859, when The London Pneumatic Despatch Company proposed a scheme of
railways in tubes under London, operated by compressed air, for the
delivery of parcels. This eventually became the original (1863) Post
Office Railway. The author of "Engineering Facts and Figures for 1864"
referred to "the contract for the tube railway".

When the City and South London Railway (now part of the Northern line)
opened in 1890, the Penny Illustrated Paper and Illustrated Times of
Nov. 8th 1890 described going "through the tube under the River Thames".

But it was probably just after the opening of the Central London Railway
in 1900 that the expression became widely used.
--
Paul Terry