The A4 locomotive is a type of steam locomotive designed by Sir Nigel Gresley for the London and North Eastern Railway (LNER) in the 1930s. Known for its streamlined design, the A4 class was built primarily for high-speed passenger services. The most famous of these locomotives is Mallard, which holds the world speed record for steam locomotives at 126 miles per hour. A4 locomotives feature a distinctive aerodynamic shape, which helps reduce air resistance and improve efficiency. A total of 35 A4 locomotives were built, and they played a significant role in the development of modern railway technology during their operational years.