Emergency services were continuing to deal with the two-carriage train, which was travelling towards Sellafield nuclear plant.
Passengers included employees on their way to work at Sellafield and officers from the Civil Nuclear Constabulary were at the scene.
No injuries were reported and, although the train left the tracks, it remained upright.
A second train sent to the scene to transport the stranded passengers to Sellafield left at 10.25am, but it too encountered a landslide and had to return to Nethertown. Passengers will now be escorted to the nearest road for their onward journey.
Network Rail said the derailment, about a mile south of St Bees station, happened in a remote area with no easy road access. Emergency services plan to remove rubble, put the train back on the rails, examine the track for damage and carry out repairs. It is likely the line will be closed for the rest of the day.
Network Rail said: "Following a landslip on to the railway near St Bees, buses are replacing trains between Whitehaven and Sellafield. Approximately 100 passengers have been removed from a train which ran into the landslip at 6.45am, derailing the front set of wheels."
From The Guardian