The Great Northern Railway 521 Class was a class of 0-6-0 steam locomotives, introduced in 1911. They were designed by Henry Ivatt for goods traffic. From 1912 to 1922 further examples, slightly modified by Nigel Gresley, were built and designated 536 Class. The most obvious difference was in the front sandboxes. These were below the running plate on the 521 but above it, and merged with the front splashers, on the 536. The boiler and firebox were also moved back, thus resulting in a shortened cab. The London and North Eastern Railway classified them both as J6.