To provide high starting torque three 16.5 in diameter 28 in stroke cylinders were employed driving the centre coupled axle, the outer pair being controlled by Walschaerts valve gear. To provide effective braking all coupled wheels were provided with a high percentage braking force operated by steam plus the unusual use (for a British railway) of brakes on the bogie. Vacuum braking apparatus was also installed for controlling fitted vehicles. The same boiler as employed on the classes N, N1 and U1 was used working at 200lb psi and employed superheating. At 85 percent of working pressure the resulting tractive effort was 29,376 lb. The engines weighed 90 tons 14 cwt in working order carrying 2000 gallons of water and 3.5 tons coal. The driving wheels were of 5ft 6in diameter and a noted feature of the class was the heavy footsteps to the rear of the outer cylinders which gave ready access to the tank fillers.

The first five engines numbered 1911-1915 (renumbered to 31911-31915 by BR) were built at Eastleigh in 1932. These emerged in black livery with green lining and yellow lettering. This first batch was right hand drive and had gravity sanding arrangements until converted to steam by BR in 1959/60. A further ten, 1916-1925 (31916-31925) were built at Ashford between 1935 and 1936. This batch emerged in unlined black livery (the first five later being repainted to conform), were left hand drive and had steam sanding from new.

The W class was another type which was unglamorous and unsung yet spent its entire life doing what it was designed to do reliably, efficiently and effectively. Being a dedicated freight engine it was rare to see a W even working empty passenger stock. However in order to compare its performance with LMS 2-6-4Ts employed on the Central and Eastern sections as part of the locomotive exchanges in May 1948 31918 was selected to work special passenger trains between Ashford and Tonbridge and between Victoria and Tunbridge Wells West. However these are the only known regular passenger turns worked by the class. Ws were based at Hither Green, Norwood Junction and Stewarts Lane for most of their lives.

The class spent its entire life in some form of black livery and their appearance hardly changed, the notable modification being the removal of the smokebox snifting valves as happened on other Maunsell types. The first member of the class 31923 was withdrawn in 1963, the last 31914 in 1964. Regrettably none is preserved.