The LNER A4 is a class of steam locomotive designed for high-speed passenger service in the United Kingdom. Built by the London and North Eastern Railway in the 1930s, these locomotives are known for their streamlined design, which helps reduce air resistance. The A4 class includes several famous engines, such as Mallard, which holds the world speed record for steam locomotives. The A4 locomotives were primarily used on mainline routes, connecting major cities like London and Edinburgh. With a powerful 4-6-2 wheel arrangement, they were capable of reaching speeds over 100 miles per hour, making them some of the fastest trains of their time.