PHILADELPHIA (CBS)— A SEPTA train crash and derailment in Upper Darby this morning left four people injured and Market-Frankford Line service temporarily suspended at 69th Street Station.

PHOTOS: Chopper 3 Over The Scene

SEPTA says the incident, involving 18 cars, happened around 8 a.m. The NTSB is now investigating.

Investigators say SEPTA El car 57 plowed into the back of car 67 at the height of Tuesday’s rush hour. Car 57 then sideswiped train number 51 which was headed in the opposite direction.

The cars derailed and leaned over the tracks.

“A total of three 6-car trains were involved — four cars from train 57 derailed, three cars from train 67 derailed,” said Scott Sauer, SEPTA’s assistant general manager.

The operator of train 57 was critically injured. He has not been identified.

Three others were also hurt, including the operator of train 67, which was stopped in the loop, on approach to the 69th Street Terminal.

“What we found when I arrived on location is four people were in fact injured – two suffered leg injuries and two suffered head injuries and we took them to two area hospitals,” said Upper Darby Township Fire Deputy Chief James Johnson.

It’s unclear why the passengers were still on board, because the train wasn’t in service.

The cause of the crash still isn’t clear, but sources tell CBS 3 it could possibly be a signal issue.

The moments leading up to the crash have not been released. The authorized speed limit in the loop is 10 mph.

SEPTA hasn’t revealed if the crash was possibly operator error or mechanical in nature.

SEPTA adds this is not related to the cracks on the Market-Frankford Line trains found a few weeks back.

BREAKING: Of the four injured, SEPTA says a male conductor is currently in critical condition. @CBSPhilly pic.twitter.com/qO5Gtv8yqi

SEPTA says Market-Frankford Line train service from 69th Street Transportation Center has resumed.

No A/B express service will operate during the evening rush hour. Passengers should expect minor delays and crowded conditions.

Regular Route 101 and 102 trolley service has also resumed between 69th Street Transportation Center and Fairfield Avenue.