in discrete modules housed behind an aerodynamic skirting between the bogies; on the Mark 2, these were mounted above and below the passenger seating area. The lighting and air-conditioning fittings were for the first time integrated into the ceiling panels. Other new features were the pneumatically operated automatic gangway doors triggered by pressure pads under the floor. From 1993, after fatal falls from moving trains, a central door-locking system operated by the train guard was installed to protect the passenger operated slam doors. The main difference between the HST vehicles and the loco-hauled Mark 3A relates to electrical supply arrangements. HST coaches take an industrial voltage/frequency 3-phase supply directly from an auxiliary alternator in the power car to supply on-board equipment such as air conditioning; loco-hauled vehicles take a standard 1000 V DC or single-phase AC train heat supply from the locomotive and convert it through motor generator units under the floor. These convert the train supply to 3-phase 415/240 V 50 Hz AC to power air conditioning and other ancillaries. The two types are non-interconnectable in service conditions. The other main difference is the lack of buffers on HST coaches. The later Mark 3B build provided first class loco-hauled vehicles for the West Coast Main Line. These are similar to Mark 3As, but have an improved motor alternator unit with compound-wound motor and seating derived from the Advanced Passenger Train (APT). Prototype Ten coaches were constructed to run between a pair of Class 41 power cars as the prototype HST, exploring different seating and layout options for first and standard class passengers, and evaluating different designs of catering facilities. In 1973 the prototype HST was evaluated as an 8-coach formation. The two spare coaches, 2903 and 2904, were rebuilt and redeployed in the Royal Train, where they remain. Development Initial plans for a large fleet for almost all InterCity services were amended prior to construction to provide stock for the planned HST fleet, resulting in a much smaller fleet of loco-hauled coaches for the West Coast Main Line. A much reduced number of coaches were manufactured, requiring many Mark 2D, 2E and 2F coaches to remain in service. The table below lists manufacturing variants as built, showing the quantity of each type/designation and original running numbers. See British Rail coach type codes for the meaning of RSB, TRUK, BFO etc. Usage Since 1977, the Royal Train has included some