A landslide has derailed a passenger train in the Swiss mountains sending one carriage plunging into a ravine, injuring 11 passengers.

Photos in Swiss media showed another carriage hanging precariously over the edge of the ravine in the heavily forested region of Graubuenden in mountainous eastern Switzerland.

Terrified passengers described crowding into the rear of one of the carriages in an attempt to prevent it from tipping over the edge.

"The train made an emergency stop in the middle of nowhere," passenger Stevens Bockor was quoted as saying by news website 20Minuten.

"We all rushed to the back of the carriage to put all of our weight there, so that it didn't tip into the abyss."

The eight-carriage regional train was travelling between the mountain resort of St Moritz and the regional capital of Chur when it derailed at around 12.45pm (11.45 Irish time). 

"At least three carriages were derailed. Several people were injured. The rescue and evacuation operation is in progress," police said in a statement.

"There were around 200 people on the train. Paramedics are treating passengers who are in shock," they said.

Simon Rageth, spokesman of regional rail company Rhaetische Bahn, said part of the train was "derailed by a landslide".

Four helicopters were rushed to the scene - a heavily forested hillside with little road access - to help evacuate the injured.

Swiss broadcaster RTS said scores of uninjured passengers made their way down the line to the local town of Tiefencastel on foot.

The region has been lashed by heavy rain in recent days and waterlogged earth is a common cause of landslides.