Interesting. I'd never seen pics of the old Bakerloo line trains before. Which tube stock would that have been?
The captions explain it - most of the shots are on the Watford extension, which originally ran with new motor coaches borrowed from the Central London Railway (whose Ealing extension was delayed by WW1) Bakerloo's own Watford extension stock being delayed for the same reason. Other shots are of the extension stock itself.