More than 40 people were taken to hospital after one light rail commuter train rear-ended another in San Francisco on Saturday, officials said.

Reuters reported that four people were in critical condition.

The San Francisco Municipal Railway L train ran into a K train at the boarding platform about 2:30 p.m. PT, officials said.

"This is probably one of the largest multiple-casualty incidents in recent years [in San Francisco]," said Pat Gardner, a deputy chief with the San Francisco Fire Department.

Gardner said 20 people suffered moderate injuries and another 21 were "walking wounded."

Witnesses said the westbound L train barrelled into the K train as it emerged from a tunnel connecting downtown San Francisco to the city's western neighbourhoods.

Judson True, a spokesman for the San Francisco Municipal Railway, said investigators were looking into "mechanical and human issues" that may have contributed to the accident.

Driver suffered serious injuries

The front of the L train was smashed and its operator was among the three with serious injuries. The K train suffered less damage, witnesses said.

They reported that more than a dozen people sat on benches along the boarding platform after the crash, some of them holding bloodied heads.

Rescue workers set up a triage system to isolate the most severely injured, bandaging their heads and immobilizing their necks on stretchers before they were trundled to waiting emergency vehicles.

"We thought a bomb went off," said Mike Burke, a San Francisco banker who lives near the crash site.

"Lots of people [in the trains] were still sitting in their seats with their heads thrown back, stunned," said his wife, Linda Burke.

Nine people were killed and more than 70 injured June 22 when a Metro train slammed into another train stopped on the tracks in Washington, D.C. The cause has not been determined but investigators say equipment that is supposed to detect stopped trains had failed periodically in the days leading up to the crash.

On May 8, more than 50 people were injured when a Boston subway trolley plowed into another train. Authorities say operator Aiden Quinn, 24, went through a red signal while typing a text message on his cellphone. Quinn was indicted on charges of grossly negligent operation and was scheduled to be arraigned Monday in Suffolk Superior Court. He faces three years in prison if convicted.