PHILADELPHIA – The Latest on accident involving three out-of-service commuter trains (all times local):

3:50 p.m.

Federal crash investigators are heading to Philadelphia after an accident involving three out-of-service commuter trains injured four people, including a train operator hospitalized in critical condition.

The National Transportation Safety Board says three investigators are heading to the site of Tuesday’s collision and derailment.

Southeastern Pennsylvania Transportation Authority spokesman Andrew Busch says one train rear-ended another on a loop where trains turn around to get back into service. Cars from that accident derailed and hit a third train on nearby tracks.

Busch says service is up and running on the Market-Frankford line, ahead of the evening rush.

The train that was rear-ended had passengers on board, and Busch said it’s not clear why, since it wasn’t in service.

Another train operator was treated at a hospital and released. Two passengers were hurt.

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12:40 p.m.

Officials are investigating an accident involving three out-of-service commuter trains in suburban Philadelphia that injured four people, including a train operator hospitalized in critical condition.

The Southeastern Pennsylvania Transportation Authority says the Market-Frankford Line trains were travelling near the 69th Street Transportation Center when the accident occurred Tuesday morning.

SEPTA spokesman Andrew Busch says one train rear-ended another on a loop where trains turn around to get back into service. Cars from that accident derailed and hit a third train on nearby tracks.

He says it’s unclear why the passengers were on the out-of-service train. The cause of the accident is under investigation.

Shuttle buses are running between the 69th Street Transportation Center and the 63rd Street Station.

Busch says the second train operator has received medical treatment but he’s unsure of the passengers’ conditions.