R40232 Mk3 Trailer First First Great Western 41131In order to allow running at 125 mph on Britain's railways, new rolling stock was needed by British Railways. Significant improvements over the Mk2 included new secondary air suspension between the bogies and the coach body as well as aerodynamic skirting on the underframe. Mk3 Coaches are 75ft (23m) long enabling far greater capacity than older coaches. Mk3 Coaches also incorporate disk brakes and wheel slip protection enabling faster deceleration.