1941 July, “Notes and News: New Tube Station at Highgate”, in Railway Magazine, page 329:
With the intensification of air raids on London last September, these tube platforms began to be used as an air raid shelter, and shelterers travelled by train (specially stopped there) from other stations in order to gain access.
1952 December, R. C. Riley, “By Rail to Kemp Town”, in Railway Magazine, page 836:
This tunnel assumed some measure of importance during the second world war, when it was used nightly as an air raid shelter for multiple unit electric trains which were propelled over the branch by a steam engine.