From The Times: July 8, 1922
On Monday electric trains will for the first time carry passengers into Euston, and as a link between the new and the old it is interesting to note that the roof over the platforms at which they will be accommodated is the same one as was erected in 1837 to shelter the first railway trains that used Euston as a terminus. This marks the completion of the electrification scheme for which the London and North-Western obtained Parliamentary powers in 1907. The first stage was to provide a third pair of rails between Willesden and Watford. This section was opened, with steam-driven trains, in June, 1912, as between Willesden and Harrow, and in February, 1913, as between Harrow and Watford.
https://www.thetimes.co.uk/article/electric-trains-at-euston-h5sh2vpmw
