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
http://hansard.millbanksystems.com/c...9010307_HOC_42
in a speech given by the delightfully named "Chairman of Ways and
Means" which I guessed was some old title that no longer exists but I
subsequently found that indeed it still does exist and is in fact one
of the Speaker's three deputies, currently Lindsay Hoyle MP from
Chorley.