third and final batch of AM4 was another ten units, numbered 036-045, which were almost identical to the second batch and intended for use on the Crewe-Stafford section. Design The Class 304 was an interurban electric multiple unit (EMU) train. Its design, which was principally devised to fulfil various requirements laid out by the British Transport Commission, was produced by a team headed by Chief Electrical Engineer S. B. Warder and Chief Mechanical Engineer I. F. Harrison. The units were arranged into four-car sets, consisting of: a Driving Trailer Brake Open vehicle with 82 second class seats (saloon) and a Guard's compartment; a Trailer Composite with 19 first class seats (compartment) and 60 second class seats (saloon), and two toilets; a Motor Brake with 96 second class seats (compartment) and a Guard's compartment; a Driving Trailer with 82 second class seats (saloon), two toilets, and battery equipment. One result of the Motor Brake vehicle having a compartment design was that the window layout on all vehicles was designed to suit, having a narrow window either side of each door. The body of the vehicle was constructed principally of welded steel. While 16-gauge sheet sections were used for areas such as the bodysides, ends, and roof panels, thinner sections were used in a minority of areas. On some units, fiberglass was inserted into the interior spaces present in the sides, roof, and ends, to provide both thermal insulation and soundproofing; on other units, asbestos was used instead. Additional insulation was sometimes installed beneath the floor panels and the corrugated aluminium floor plate. While much of the passenger compartment's interior finishing was a mixture of timber and plastic, light alloys were used for several elements, such as the doors and luggage racks. The electrical systems of the Class 304 were largely supplied by a single company, the British manufacturing group Associated Electrical Industries (AEI). Powered by 25 kV AC via catenary, a single pantograph mounted directly above the guard's compartment of the Motor Brake vehicle gathered current from the overhead catenary, which was transferred via a rubber insulated cable from the roof directly to the onboard transformer. This transformer, in combination with a germanium rectifier, stepped down the supplied current and converted it from AC to DC; the use of a germanium rectifier was a novel feature of the type. This system incorporated an automated power control system and voltage selection circuit.