The Watford Joint Tube Stock was built for the service to Watford along both the Bakerloo tube and the London North Western Railway. As a result, the cars were owned by both the Underground and the London North Western Railway. To be able to operate on both lines, the car floors were higher than other tube cars. This was a compromise height between the platform heights on the two lines. The cars were ordered in 1914, but construction was delayed by The First World War. As a result, the first cars were not delivered until early 1920.