LB&SCR I3 Class

The LB&SCR I3 Class was a series of outside-cylinder 4-4-2 Atlantic side-tank locomotives designed by Douglas Earle Marsh and built at Brighton Works from 1907, forming part of the extensive family of Marsh Atlantic tank types that he introduced for the LBSCR's outer-suburban and coastal express services. The I3 was a detail variant within the I-class Atlantic tank programme, with specific boiler and cylinder dimensions that distinguished it from the closely related I1 and I2 introduced in the same year.

Like all Marsh's Atlantic tanks, the I3 used outside cylinders and Walschaerts valve gear adapted from the H1 tender Atlantic, in a tank engine form suited to the semi-fast and express suburban services on the LBSCR's routes to the Sussex coast and the outer London suburbs. The proliferation of I-class variants — I1 through I7 within a few years — reflected Marsh's systematic approach to optimising the basic Atlantic tank design for slightly different traffic requirements across the LBSCR's varied passenger services.

The I3s passed to the Southern Railway in 1923 and continued on secondary express duties. None was preserved.