LB&SCR I4 Class

The LB&SCR I4 Class was a series of outside-cylinder 4-4-2 Atlantic side-tank locomotives designed by Douglas Earle Marsh and introduced in 1909, continuing the LBSCR's Atlantic tank programme with further detail refinements to the boiler and valve gear that Marsh was progressively applying across the I-class family. The I4 represented Marsh's ongoing development of the basic Atlantic tank formula for the LBSCR's outer-suburban and coastal express services, each incremental variant reflecting lessons learned from the operation of the preceding types in service.

By 1909 Marsh had been building Atlantic tanks for the LBSCR for two years and had accumulated operational experience that informed the detail improvements applied in the I4. The core Atlantic tank concept — outside cylinders on a 4-4-2T chassis for high-speed semi-fast working — remained unchanged, but boiler pressure, valve travel, and detail fittings were refined in the search for the optimum specification for the LBSCR's express suburban services.

The I4s passed to the Southern Railway in 1923. None was preserved.