This fleet of Liverpool to Southport/Ormskirk stock was constructed at Derby by the LMS in 1939/41 to replace original stock of 1904 vintage.
The line which in later years become known as the 'Northern Line' was electrified at 630V dc with pick-up via the third rail system. Stock was of similar appearance to that of the Wirral (Class 503) units. A total of 152 cars was built, 59 Driving Motor Brake Second (DMBS), 50 Trailer Second (TS), 9 Trailer Composite (TC) and 34 Driving Trailer Composite (DTC). Vehicles were formed into 34 three-car units formed MBSO, TSO, DTCO, and 25 two coach sets formed either MBSO-TSO or MBSO-TCO These formations meant that the two-car sets only had a driving position at one end and therefore had to be coupled to another set for public service. The dimensions for all vehicles was 66ft 6in Iong by 9ft 6in wide. Their construction was not totally conventional as the bodies were not carried on a frame, but the body itself acted as a load bearing tubular structure. Internal layout was in the high density styling of
2 + 3 in second class and 2 + 2 in the first class; passenger access was by air operated double leaf sliding doors. Trac tion and control equipment, all housed in the DMBS, was supplied by English Electric; third rail collector shoes were fitted at both ends of power cars but not on any other vehicle. The DMBS cars were close-coupled to the adjoining vehicle but all other coupling was of the screw/shackle type. Outer ends of trailer cars were fitted with an observation window and a signal button, used for shunting movements, these being necessary as a driving position was not provided at both ends of all fixed sets.
Normally, stock remained in the intended formations, but in later years some alterations were carried out, including coupling driving trailer vehicles to two car sets. After many years of service, the nine TC vehicles were rebuilt, some into TSOs and others into DTCs. A number of vehicles were withdrawn in the 1960s and early 1970s mainly as surplus to requirements, leaving a total of 36 three-car and 7 two-car units still in service by 1973.
The Class 502 units remained in service until 1981 when all were withdrawn. Again, in common with LMS dc unit policy, no unit numbers were carried, but formations usually remained constant for lengthy periods.