These locomotives were an improved version of the Class S56 tanks, and the final development of James Holden's Class T18 tank locomotives, sharing the same 16.5-by-22-inch (419 mm × 559 mm) cylinders, 4-foot-0-inch (1.219 m) driving wheels, and 13-foot-10-inch (4.22 m) wheelbase.

There were three orders, each of ten locomotives, all built at Stratford Works between 1912 and 1923.

The first batch were built as suburban passenger tanks and were fitted with Westinghouse air brakes.

One locomotive was lent to the War Department in October 1939,[2] and sold 12 months later,[3] It was used on the Longmoor Military Railway before being moved to the Bicester Central Ordnance Depot,[2] and then the Military Port No.

At nationalisation in 1948 they passed to British Railways, who added 60000 to their numbers.