Built to eliminate double-heading of C2X class 0-6-0s they had to have good acceleration for working heavy goods trains in the London area, thereby minimising delays to the suburban services. Nos 337 and 338 were built in 1913, 339-341 in 1914 and 342-346 in 1916. Following the success of the top feed arrangement on Nos 345 and 346, this was fitted to the earlier locomotives as and when they were required to go through the workshops. More were required but wartime restrictions led to a clamp down on further building so it wasn't until Government control was withdrawn in 1919 that more could be built. Ordered that same year, Nos 347-350 appeared in 1920, followed by the last of the class, Nos 351-353 in 1921. At one time a 2-6-2 tank version, at first designated Class F*, then later Class K2, was proposed, but the final batch of tender locos was built instead**.

As with the C2X class some of the locomotives were fited with a second dome housing the top feed. This raised the clacks and allowed a deeper distributing tray which prevented suspended matter in the water from entering the boiler.

Designed as a Heavy Goods Locomotive, the class spent much of its early years working munitions trains to Newhaven. In order to accomodate them there a brand new, 60 foot, ball-bearing turntable was built at Brighton Works and installed at Newhaven in 1917.

Among other noted innovations in 1921 N°351 was fitted with a Lewis draught regulator, which required the smokebox being extended, and, for a time, a ship-like plain oval chimney. Also in 1921 N°340 was fitted with a Worthington feedwater heater and pump (which the Southern Railway removed in 1926). N°347 differed from her sisters in that she was fitted with 'pop' safety valves.

In Southern Railway days N°2342 was fitted with electric lamps, the first coal-fired locomotive apart from the Bulleid Pacifics to be so equipped. The generator was fitted to the right hand cab front behind the Westinghouse pump. This equipment was subsequently removed by British Railways in 1951.

  * One source suggests the tanks were to be Class M.
** One source suggests this happened in 1916 and that it was the Government who requested that tender engines be built.