Six hundred Tube passengers have been moved to safety after a landslide near a station.
Several tonnes of earth slipped onto the track
The train at Boston Manor station, west London, was evacuated after debris landed on the tracks.
A Piccadilly Line train made an emergency stop before hitting debris, but no-one was hurt and the train stayed on the tracks.
The Piccadilly Line has been suspended between Northfields and Heathrow while the damage is inspected.
LU said all passengers were led off the train onto the platform at Boston Manor within 15 minutes.
A spokeswoman said: "London Underground is currently assessing the impact of an embankment landslip between Boston Manor and Northfields.
"Initial indications are that three to four tonnes of debris has ended up on the track.
"An eastbound Piccadilly Line train came into contact with the debris at Boston Manor station."
A replacement bus service is in operation between Northfields and Heathrow and Tube tickets can be used on Heathrow Connect rail services from Ealing Broadway.
LU staff are trying to clear the debris overnight in time for services to resume on Friday morning.