In 1895, the C&SLR itself built locomotive No. 17 at Stockwell depot, and carried out a series of tests on locomotives 12, 15 and 17, as more locomotives would soon be needed for the extensions being made. They ordered three more locomotives from different manufacturers in 1898, which were equipped with four-pole motors, a more efficient control system using series-parallel switching of the motors, and on-board compressors. The motors were still mounted on the axles. Another two locomotives (Nos. 21 and 22), which were built at Stockwell Depot, included further refinements and were the prototypes for the final batches of locomotives. Nos. 23 to 52 were built by Crompton to an improved design, including nose-suspended motors connected to the axles by a single reduction gear, but still bore a strong external resemblance to the original locomotives. Between 1904 and 1907, locomotives Nos. 3 to 12 were rebuilt with new electrical equipment to improve their performance.
