The SECR K and SR K1 classes, also referred to as the River class, was a type of 2-6-4 tank locomotive designed in 1914 by Richard Maunsell for express passenger duties on the South Eastern and Chatham Railway (SECR). Built between 1917 and 1925, it was the second non-Great Western Railway (GWR) type to use and improve upon the basic design principles established by GWR Chief Mechanical Engineer (CME) George Jackson Churchward. The K class was based on the GWR 4300 class, improved with Midland Railway concepts. The K class was mechanically similar to the contemporary SECR N class 2-6-0 mixed-traffic locomotives and influenced future 2-6-4 development in Britain, providing the basis for the three-cylinder K1 class of 1925. The class was the earliest large-scale use of the 2-6-4 wheel arrangement in Britain. It also provided the basis for the Southern Railway's three-cylinder K1 class of 1925. Production began towards the end of the First World War, and the first K class rolled out of Ashford Works in 1917, three years after design work was completed. The class replaced obsolete 4-4-0 passenger locomotives as part of the SECR's fleet standardisation, as they made use of parts interchangeable with other classes. Twenty-one K and K1 class locomotives were built between the First and Second World Wars. They operated over the Eastern section of the Southern Railway (SR) network and were given the names of various rivers. Crews referred to the K and K1 classes as "Rolling Rivers" because of their instability when traveling at speed. Both classes were rebuilt as SR U class and SR U1 class 2-6-0s following the 1927 Sevenoaks railway accident. They continued in service with British Railways (BR) until the last was withdrawn in 1966. One K class rebuild is preserved on the Watercress Line in Hampshire, and is, as of 2009, under overhaul.
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