The slip happened when earth gave way along a nine-metre section of embankment, sliding out from under the tracks.
Project manager, Nad Campbell, said the repair job involved removing 2,800 tonnes of material.
Damaged track had to be taken away, the embankment stabilised and a concrete platform built capable of holding a 50‑tonne machine.
The slope was then reshaped to make it stronger.
Campbell praised the team effort as “fantastic”.
Network Rail Sussex route director, Lucy McAuliffe, described the landslip as “significant”.