bogie. They are fitted with trip-cocks that are struck by a raised train-stop arm at red signals and will apply the brakes if the train passes one. The units were originally numbered 313001–313064. Each unit is formed of two outer driving motors and an intermediate trailer with a pantograph. This is a reversal of the practice started in the 1960s, where the motors and pantograph were on an intermediate vehicle, with the outer vehicles being driving trailers. Part of the reason was to simplify the equipment to allow dual-voltage operation, and to keep down weight by spreading the heavy transformer and motors between vehicles. The intermediate trailer carries the pantograph and a transformer and rectifier, which on 25 kV AC provides 750 V DC to the motor coaches, each of which has four GEC G310AZ direct current traction motors, two per bogie. On 750 V DC each motor coach draws its supply directly through its shoe gear. The traction motors are driven by a camshaft-controlled resistance system with series and parallel motor groupings and weak field steps. Originally the heating in the motor coaches was provided by passing air over the hot traction and braking resistors in addition to conventional heaters, but this feature is no longer in use and the pneumatic dampers have been disabled. Great Northern and Southern retrofitted their units with cab air conditioning. 313s have rheostatic braking (which was disabled on London Overground) in addition to conventional three-step air-operated disc braking. During braking if wheelslide is detected by the Wheel slide protection (WSP), rheostatic braking is disabled and disc-braking only is used. Great Northern units had sanding equipment. Unlike some other DMU/EMU classes, additional brake force is not available when the emergency brake application is initiated and is the equivalent force of a step 3/full service application. WSP is still active when making an emergency application. In addition to the primary suspension of rubber chevron spring and oil dampers, secondary suspension is provided by two air bellows per bogie - flow into each bellows is controlled independently by a levelling valve and arm assembly that allows the suspension to inflate/deflate when the weight of the coach is increased or decreased by passenger loading. The air suspension is linked to the braking system via a Variable Load Valve (VLV), which increases air brake pressure when the coach is more heavily loaded to compensate for the additional weight.