After successful testing of the prototype high density stock (PEP)on the Southern Region, production sets of various types were authorised. By the late 1970s, the SR were getting desperate for new rolling stock and were given the 43 strong Class 508 fleet as a stop-gap until purpose-built SR replacement stock was made available. The Class 508 units were four car sets and formed of a Driving Motor Second (DMS), Trailer Second (TS), TS and DMS. The basic design followed the PEP with both coach and internal layout, and sliding doors were again fitted, but with passenger push button control. Power equipment, supplied by GEC, in the shape of one 110hp traction motor to each axle was fitted to the driving cars only. Braking was the standard EP type with additional rheostatic (dynamic) brakes on power cars. The passenger accommodation, set out in the high density style, provided seats for 74 in DMS and 84 in TS cars. However, coaches were designed to take an equal number of people standing.
The first Class 508 units were delivered to the SR in the autumn of 1979, and after commissioning and training, commenced passenger operation from December 17, 1979, progressively replacing aged suburban units as they were delivered. After a short period of service, problems were encountered with wheel slip/slide protection equipment, which reduced brake pressure or cut off power if any indication of wheel slip/slide was detected by the unit. Unfortunately, this equipment was not totally satisfactory, and for a lengthy period set No. 508002 operated under test conditions to seek a remedy for the problem, which was eventually overcome by fitting a wheel slip protection override button.
Class 508 livery was blue/grey, from new, with full yellow warning ends and black cab window surrounds. Another break from previous SR practice was the omission of the two digit route indicator and the fitting of a route destination panel.
By November 1981, the SR stock position eased with delivery of Class 455s, and two units, Nos. 508042/3 were reduced to three car formations and transferred to the London Midland Region at Birkenhead to operate on their intended Merseyrail electrified route. Sets Nos. 508039-41 followed in February 1983, and the remainder of the fleet by December 1984. The sets were then allocated to Birkenhead and worked as a three car formations on Liverpool suburban duties, working alongside Class 507 sets. For their MerseyRail role, the sets were later repainted in yellow/white livery.
After spare Class 508s were identified on Merseyside, 12 sets were made redundant and transferred via facelift to Connex for Kent Coast branch line use, for this application the sets were re-classified as 508/2.