The A4 class was designed by Nigel Gresley to power high-speed streamlined trains. The wind-tunnel-tested, aerodynamic body and high power allowed the class to reach speeds of over , although in everyday service it rarely attained this speed. While in British Railways days regular steam-hauled rail services in the UK were officially limited to a 90 mph 'line speed', pre-war, the A4s had to run significantly above 90 mph just to keep schedule on trains such as the "Silver Jubilee" and "The Coronation", with the engines reaching 100 mph on many occasions. "Mallard" covered almost one and a half million miles (2.4 million km) before it was retired in 1963.