 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 25 miles per hour ( 40 km / h ) . Standard 9 feet 0 inches ( 2 @.@ 74 m ) 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 Railways ' 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 . 
