One passenger died and 42 were injured on Friday when one of Switzerland's best-known tourist trains, the Glacier Express, derailed at the height of the holiday season, a railway official said.
Swiss media said most of the injured were Japanese tourists travelling in first-class carriages at the rear of the train which left the track. Twelve were in serious condition.
The Glacier Express travels from the resort of St. Moritz through spectacular Alpine scenery to Zermatt, which is dominated by the Matterhorn.
The accident took place shortly before midday as the train, carrying some 210 passengers, was going over a viaduct between the ski resort of Lax and the village of Fiesch, near the Swiss-Italian border. Two carriages toppled over, and a pylon stopped a third which had derailed.
More than 150 workers were immediately deployed as part of an emergency rescue team comprising helicopters, ambulances, doctors and firemen who rushed to the scene, officials said.
"We felt a shock and saw several train cars derailed on the curve," a Swiss passenger told Swiss television.
Jean-Marie Bornet, spokesman of the Valais canton police, said the train was not moving fast and was on a gentle curve when the accident occurred.
"It is surprising that the train derailed at this spot," Bornet told Swiss television. "There is no explanation yet for this tragedy." Fatal train accidents are rare in Switzerland.