The bodyshell is long, almost longer than the Mark 2, of full monocoque construction with an all-welded mild steel stressed skin, and has a reputation for its exceptional strength and crashworthiness. An important advance over its predecessor was the adoption of secondary air suspension between the body and the bogies, giving an exceptionally smooth ride. The bogies, classified BT10 (BT5 on the prototype vehicles), were designed specifically for the Mark 3 and have coil-spring primary suspension with hydraulic dampers, enabling a maximum speed of – the Mark 2 is limited to . Disc brakes in place of the Mark 2 clasp brakes completed the engineering package enabling – in conjunction with wheel slip protection (WSP) – efficient deceleration from 125 mph and almost silent brake operation.