Efforts to get the Market-Frankford Line back to full strength faced another setback Tuesday morning when a derailment took three trains out of service.

SEPTA's most heavily traveled service, which moves about 187,000 riders daily, was already diminished earlier this month after inspectors found cracks in a key load-bearing component in the nearly 20-year-old rail cars. As of Tuesday, cracks had been found in the body bolsters on three cars, and 44 cars had been pulled from service due to concerns about other cracks forming.

Tuesday's derailment will at least temporarily sideline 18 more cars, authorities said. The derailment was not caused by the cars' structural woes, said Scott Sauer, SEPTA's assistant general manager for system safety, but "it certainly doesn't help." SEPTA's regular weekday service requires about 24 trains. With Tuesday's derailment, it is down to 19.

The incident happened about 8:10 a.m. on a loop beyond the 69th Street Transportation Center in Upper Darby, according to SEPTA.

Train 67 was stopped on the slow curve, waiting to be cleared to head to Center City. Train 57 then ran into the stopped train, and as it toppled, its cars collided with Train 51 on an adjacent track, Sauer said.

Train 67 was cleared to stop on the loop, Sauer said. Subway operators have dashboard indicators designed to alert operators to a stopped train ahead, as well as red-light signals alongside the tracks.

The cause of the derailment is under investigation by the National Transportation Safety Board, said Terry Williams, a spokesman for the federal authority. Three investigators were expected to arrive Tuesday night.

For more than five hours Tuesday morning, shuttle buses replaced trains between 69th and 63rd Streets before the line resumed full service around 1:30 p.m. SEPTA was not running express service on the El Tuesday night because of the car shortage, said Andrew Busch, a SEPTA spokesman. The Delaware Valley Association of Rail Passengers warned of crowding and delays on the El during rush hour.

The operator of Train 57 suffered serious injuries and was listed in critical condition at Lankenau Hospital on Tuesday night. His name was not released and representatives Transportation Workers Union Local 234 did not reply to calls about the operator's condition and history with SEPTA.

The operator of Train 67 and two passengers suffered non-life-threatening injuries. The operator of Train 67 has been released from the hospital. The trains involved were between runs and were not supposed to be carrying passengers, authorities said. It was not immediately clear why the riders were on the trains.

Tuesday afternoon, SEPTA workers cut away fences to give heavy machinery better access to the affected trains. Seven cars were knocked off the tracks by the impact, and a crane would be used to hoist the approximately 30-ton cars back onto the rails so they can be moved. It was not clear Tuesday how many of the 18 cars involved would require repairs.

In the first week of February, dozens of cars on the line were pulled from service after the discovery of cracks in vent boxes. In three cases, the cracks extended into a load-bearing part called the body bolster, a lateral beam that connects to the wheel assembly beneath. The cracks appear to have been caused by a faulty weld.

Derailments are exceedingly rare on the city's subway lines. The El's worst accident came in March 1990, when four died and 158 were injured in a derailment. Prior to that, the last derailment on the line was in 1961.