 Electric services were introduced over the Watford and Rickmansworth Railway on 16 April 1917 worked by tube trains of the London Electric Railway (LER) running through from Queen's Park to Watford on weekdays only until a daily service was introduced in July 1919; this was done to cope with the voltage drop caused by the branch being supplied only from the Watford end.These services were supplemented by LNWR trains from Broad Street during peak periods and steam trains from Euston.The Croxley Green branch was electrified on 30 October 1922, with Rickmansworth following in September 1927 as part of the LNWR's New Line Project.