In October 1946, the Southern Railway announced a programme to electrify all lines in Kent and East Sussex in three stages. The Hastings line between Tonbridge and Bopeep Junction was to be part of the third stage. Track would have been slewed within the affected tunnels with only one train normally allowed in the tunnel. In an emergency, two trains would be allowed in the tunnel at the same time, but restricted to . Standard wide stock would be used. Following the nationalisation of railways in the United Kingdom under the Transport Act 1947, the Southern Region of British Railways shelved new electrification schemes, concentrating on the construction of new steam locomotives. In 1952, the possibility of operating standard rolling stock on the line had been examined. The Operating Department objected to the use of single line sections through the various tunnels. The 1930s stock was refurbished with the aim of extending its service by a further ten years. The first two phases of the Southern Railway's electrification scheme were revived in 1955. This did not include the Hastings line and it was announced in 1956 that a fleet of diesel-electric trains would be constructed to operate the service until the line was electrified. At that time, the rolling stock built in the 1930s was overdue for replacement. The modernisation to the Hastings line and the introduction of the diesel-electric trains cost £797,000, of which £595,000 was the cost of the first seven trains. A further thirteen trains cost £1,178,840.