Saltley (who built the motor coaches), and Birmingham RC&W of Smethwick (who built the underframes). Both companies had previous experience of building this type of train for the London Underground. Significant points of similarity can be seen between the Class 503 and the London Underground O and P Stock, produced shortly beforehand by Birmingham RC&W. The traction motors were supplied by British Thomson-Houston. Operation Between 1936 and 1938, the Wirral lines were modified and electrified using a 650 V DC third rail system. The new stock was used on the West Kirby line on weekdays, and the New Brighton and Rock Ferry lines on Sundays. including inter-running onto the Mersey Railway through to Liverpool, while the older Mersey Railway electric units were now used on the New Brighton route, during the week. After the 1956 stock was built, it was normal for the West Kirby route to be operated by the newer stock, and the New Brighton/Rock Ferry routes to be operated mostly by 1938 stock. When the Liverpool loop lines were opened, the stock became fully mixed on all routes. The units spent most of their working lives on the commuter routes between Liverpool and the Wirral; routes which latterly became known as the Wirral Line of Merseyrail. In 1977, three six-car trains were transferred to the Northern Line. They mainly worked the Kirkby to Garston service until 30 October 1981, and were then returned to the Wirral Line. Wartime destruction and replacements On the night of 12–13 March 1941 four of the 1938 cars, two trailers and two driving trailers, were destroyed by wartime bombing while standing at Birkenhead North in the same air raid which severely damaged Birkenhead Park station, and destroyed much else of Birkenhead and Wallasey. The two associated motor cars remained spare until the 1956 batch was built, when four replacement cars were added to the production. This accounts for the unbalanced number of cars produced in 1956. A known feature of the Ian Allan ABC rolling stock books for many years was that the destroyed cars continued to be listed, incorrectly, until well into the 1960s and after the replacements had been built. Interiors Loading gauge restrictions meant the class did not have the width, or length, which was possible with the Class 502 Liverpool to Southport sets introduced shortly afterwards, to a somewhat similar design. Seats in second class were 2+2, and