UPPER DARBY, Pa. (CBS) — A train operator was critically injured following a SEPTA train crash and derailment Tuesday morning.

Scott Sauer, SEPTA’s assistant GM of system safety, says one train going around the loop rear-ended a train that was stopped on the tracks.

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SEPTA Trains Derail Near 69th Street Terminal

The train cars derailed, leaning over the tracks. They sideswiped the third train involved, which was heading in the opposite direction on a different track.

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Officials say two SEPTA train operators are hurt, one critically, and two passengers as well. It’s unclear why the passengers were still on board, because the train wasn’t in service while looping around.

The cause of the crash isn’t clear, SEPTA says the max speed allowed on the loop is 10 miles per hour.

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The accident caused the Market-Frankford Line service to be suspended at 69th Street Station, but it has since resumed.