However, this process was interrupted by the Second World War, and although much of the Standard Stock had been through Acton Works for refurbishment, it was stored, as the opening of the extensions was delayed.

Further refurbishment was carried out as the extensions opened between 1946 and 1949, but the stock proved to be unreliable after such a long period of storage.

The British Transport Commission approved the project "in principle" in 1950, and a design contract was awarded to Metro-Cammell.

The high cost of building the trains, and a decline in passenger numbers, resulted in the plans being shelved in September 1952.

[2] When the situation was re-assessed in 1954, London Underground decided to build three seven-car trains incorporating new ideas, and these became the 1956 Stock.

[4] While they were similar in many respects to the 1938 stock, the main differences were that the body panels were unpainted aluminium, rather than painted steel, a rubber suspension system was used on the bogies, reducing the number of parts that required regular maintenance, and the lighting was provided by fluorescent tubes, rather than incandescent bulbs.

The electro-pneumatic brake was of a similar design to the 1938 stock, but the motor generator set was rather different, as it needed to supply alternating current at 110 V and 850 Hz.

The design was influenced by problems with the fluorescent lighting supply on the sub-surface R stock, which tended to generate a loud magnetic hum.

The weak field flag switch allowed higher speeds still, but was only to be operated on open sections of the line.

When first delivered, Wedglock automatic couplers were fitted to the motor cars in the middle of the train, which provided mechanical, pneumatic and electrical connection.