By 1938, the Waterloo a City line had been in operation for 40 years and the Southern Railway decided to refurbish the line and introduce new stock. English Electric at Preston were awarded a contract to build 12 double-ended motor cars and 16 trailers. These new vehicles, each measuring 49ft.1 3/4in long, were fitted with air operated sliding doors. Passenger accommodation was for 40 in motor cars and 52 in trailers, and large gangways were provided enabling a considerable number of standing passengers to be transported.
All traction and control equipment was supplied by English Electric and was mounted at one end of each motor car (the Bank end); each motor car being powered by two 190hp traction motors.
The formation of trains was usually five coaches (a motor coach at each end with three trailers between) however, during off-peak periods, two motor coaches could be coupled together to operate as a power twin. Technically, a single motor coach could operate on its own, but due to operating restrictions this was not usually permitted.
To operate a full service, four five car sets were required, and these were identified by letters carried on the nose end of sets. All regular maintenance was carried out in sidings at the Waterloo end of the line, with heavy and classified overhauls being dealt with at Selhurst, Wimbledon or Eastleigh.
One non-standard feature of the stock was that it carried red marker lights at both the front and rear when in service. Numbering of motor cars was S51-62 and trailers S71-86. When built, livery applied was SR green with silver ends, after the late 1960s this gave way to BR rail blue with grey ends. After the 1980s formation of Network Southeast, the Waterloo& City line formed part of this business and the stock was repainted into the new house colours.
Modernisation of the Waterloo & City line took place again in 1993, when as part of a total rebuilt of the line between Waterloo and Bank new stock was provided, this time it was built as a follow-on order of London Underground Central line stock. The 1940s built sets were all removed from the line by crane in summer 1993 and new stock lowered down to the line. One driving car is now preserved at the National Railway Museum, York.
Soon after the 1993 modernisation the line was sold from BRB (NSE) ownership to LUL and is now operated by London Underground as part of their Central Line operation, but the route still has its own branding and livery.