SECR K and SR K1 Classes - Design and Construction - K1 Class

K1 Class

In August 1919, a proposal was put to the SECR’s Locomotive, Carriage and Wagon Committee for 2 and 3-cylinder tank engines of 2-8-0 wheel arrangement for heavy shunting of freight wagons. They were to use the same boiler as the K and N classes and the general layout was similar to designs used by the GWR in South Wales. Nothing came of this proposal because of other commitments and the absorption of the SECR into the Southern Railway. In 1922, Holcroft suggested that 3-cylinder 2-6-0 tender locomotives with 6 ft (1,830 mm) driving wheels should be built instead of tank locomotives. Despite the benefit of a greater operational range, Holcroft's immediate superior, Clayton, refused to pass this suggestion on to Maunsell. The 3-cylinder principle was therefore applied to the K class.

At the Southern Railway’s January 1925 Locomotive Committee meeting, when it was decided to use outside contractors to build the K class, Maunsell received authority to retain one set of parts at Ashford works to construct a prototype 3-cylinder 2-6-4 tank. The modification was based upon that used on N class No. 822 to produce a 3-cylinder locomotive in 1922, although it retained the 6 ft (1,830 mm) driving wheels and shorter wheelbase of the K class. The modification was the inclusion of an additional (inside) cylinder between the frames, and a crank axle was fitted to the middle driving wheels. The axle was connected to the inside cylinder assembly by a connecting rod inclined at 1 in 8 to clear the front driving axle. This arrangement was supplemented by two smaller-diameter outside cylinders with 16 in (406 mm) bore (compared to the 19 in (483 mm) cylinders of the K class), and a greater chimney diameter. The resulting prototype 3-cylinder "K1" was narrower than the K class and hence could work on routes with restricted loading gauge. As with No. 822, this locomotive used Holcroft's derivative of the Gresley conjugated valve gear to drive the inside cylinder. To accommodate this, the boiler had to be raised by 3 in (76 mm) above the inside gear, raising the centre of gravity on the locomotive.

The main visual difference between the K and K1 classes was at the front end: the K1 incorporated a vertical metal cover above the front buffer beam to protect the third cylinder and associated Holcroft valve gear assembly from the elements. It also featured a new cab design with redesigned single front spectacle plates, and a pair of substantially constructed steps were fitted behind both outside cylinders to provide access to the running plate. The lack of a middle cylinder on the K class locomotives had allowed the provision of a footplate that curved from the buffers to the water tanks. The K1 prototype emerged from Ashford works as No. A890, and underwent trials from 1 December 1925 before entering regular service. Only one locomotive of the K1 class was built; plans to build a further ten (Nos. A891-A900) alongside a batch of five N1 class 2-6-0s were cancelled after the Sevenoaks accident in August 1927. Following rebuilding as a 2-6-0 tender locomotive in 1928, No. A890 was reclassified U1 and was the forerunner of twenty more basically similar locomotives built in 1931.