Two light rail cars collided in San Francisco on Saturday, injuring 48 people, four of whom were in critical condition, officials said, according to Reuters.

The mid-afternoon accident caused service delays into the evening between the West Portal and Castro stations in California`s third largest city.

Initial reports indicating that a single-car train was traveling at low speed when it collided with a stopped train have not been confirmed, San Francisco Municipal Transportation Agency (MTA) spokesman Judson True said in a statement.

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The investigation of the accident is ongoing, True said.

There have been several such accidents in the United States in recent months.

Last month nine people were killed and more than 70 injured when a commuter train in Washington D.C. slammed into another that had stopped during the afternoon rush hour. The cause of the crash has not been determined.

Twenty-five people were killed and 130 injured last year when a Los Angeles commuter train collided with a freight train.