Electrification was extended north from Harrow to Rickmansworth, and branches opened from Rickmansworth to Watford in 1925 and from Wembley Park to Stanmore in 1932.
Elektrificeringen blev forlænget mod nord fra Harrow til Rickmansworth, og afgreninger åbnede fra Rickmansworth til Watford i 1925 og fra Wembley Park til Stanmore i 1932.