SAN FRANCISCO (AP) Authorities in San Francisco are looking at mechanical and human errors as possible causes of a collision between two light-rail trains that left several dozen people hurt.

Transit officials are promising a speedy investigation to get to the bottom of the crash so that such an accident does not happen again.

Authorities say 48 people were taken to hospitals four with what appeared to be severe injuries.

Witnesses said the westbound L train barreled into the K train at the West Portal Station as it emerged from a tunnel connecting downtown San Francisco to the city's western neighborhoods.

The operator of the L train was among the four with serious injuries.

Saturday's crash was at least the third major transit accident in the United States since May.

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