In October 1913 Billinton ordered the design of an enlarged version of the J class tank, with the trailing truck replaced by a bogie. This 4-6-4 or Baltic wheel arrangement was intended to be more stable at speed than the 4-6-2 arrangement of the original design. An order for five locomotives was placed with Brighton Works in November 1913, and the first, no. 327, was completed in April 1914. Construction of the second member of the class, no. 328, was interrupted whilst no. 327 was tested. As at the same time Billinton began a project to design and construct an express 4-6-0 tender locomotive, it has been suggested that he may have been intending to complete no. 328 as such. No. 327 was involved in a couple of minor derailments in August and November 1914, which were ascribed to the surging movement of water in half-full side tanks. The locomotive was withdrawn from service whilst consideration was given to its conversion to a tender engine. Billinton instructed the drawing office in February 1915 to cost both a conversion scheme and a scheme to modify the side tanks to improve stability. As costs appeared to be almost equal it was decided to complete no. 328 as a tank engine, and to modify both locomotives by inserting a well tank between the frames and restricting the depth of water in the side tanks. By the time no. 328 was completed in March 1916 the order for the remaining three locomotives had been cancelled because of wartime material shortages, and nos. 327 and 328 remained the only members of the L class until 1919.