The 4EPB units (4-car Electro-Pneumatic Brake) were a development of the Southern Railway (SR) 4Sub design, but incorporating electro-pneumatic brakes, unit-to-unit buckeye couplings, roller blind headcode displays in place of the stencil holders used previously, a revised front and without external doors to the driver's cab – access was via the adjacent guard's compartment.

Two units differed from the rest of the batch in featuring B5 (S) bogies to enable use on peak hour commuter trains to Eastbourne.

In the earlier, Southern-style 5001-5053, 5101-5260 series, most units (all of which were one class only) comprised a driving motor open saloon including brake at each end of the set, sandwiching a trailer open and a high-density (6 per side) trailer ten-compartment vehicle with access from the passenger doors only; there was no gangway down the coach.

A small number of the trailers had been built as 'composites' – a mixture of First and Third Class – and were later fitted out as 9-compartment one-class vehicles but with the former 1st accommodation still identifiable with extra-wide compartments.

The production vehicles in the BR series 5301-5370 had slightly higher capacity motor coaches, identical vehicles at each end of the set, with an internal partition splitting the saloon into two smaller ones, and a pair of identical trailers each comprising 5 compartments and a 5-bay open saloon, with the compartment end of each coach always back-to-back with its neighbour.

However, on 23 March 1988, a woman was found murdered in a compartment EPB car on an Orpington/London Victoria working which led to Network SouthEast reconfiguring the then-remaining unrefurbished SR-design 4-EPBs; as a result all compartment stock ran limited workings in busy periods and had a red stripe at the cantrail (the place where the bodysides meet the roof).

Some of the 56xx series units received express gear ratios to allow them to work services between London and Kent Coast destinations.

ceiling bulbs lit each compartment, and the partitions were initially painted in light cream: three publicity panels about 15 cm x 40 cm (6" x 16") filled the space on the compartment walls between the top of the seats and the luggage racks – the central panel was originally a mirror, but as these proved dangerous in service they were soon replaced with normal advertising.

The other fittings were a chain alarm-cord in a recess above and to one side of the door, and the door-lock itself, a simple spring-loaded slide: the slam-doors could be opened at speed, albeit at considerable risk to the passenger doing so.

Underneath was a small worksurface about 2'x1', a swivel-chair screwed to the floor, and variously a short ladder for evacuating coaches, sometimes a medical cabinet, and a rail-shorting bar would be fixed to the side of the compartment.

Class pioneer 5001 was also stored at Kineton Ministry of Defence base following withdrawal in 1995 but no buyer could be found and the unit was stripped and sold for scrap in 2004.