SAN FRANCISCO -- 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.