A train on SEPTA's Market-Frankford Line derailed Tuesday in a rail yard when it crashed into a stopped train, seriously injuring one of the operators.

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

Train 67 was stopped on the loop, waiting to be cleared to head east back into Center City. Train 57 then ran into the stopped train. Then, cars on that train then collided with Train 51 on an adjacent track, said Scott Sauer, SEPTA's assistant general manager for system safety.

The male operator of Train 57 suffered serious injuries and was listed in critical condition at Lankenau Hospital in Wynnewood. His name was not released. The operator of Train 67 and two passengers suffered non-life-threatening injuries.

The transit agency said the accident involved "non-revenue Market-Frankford Line trains," indicating they were not carrying passengers. It wasn't immediately clear why the riders were on the trains.

The area where the crash happened was described a slow curve.

Normal service on the subway line was resuming by mid-afternoon, though SEPTA warned that riders might face delays or crowding.

For more than five hours, shuttle buses had been replacing trains between 69th and 63rd Streets before resuming full service around 1:30 p.m. This was the scene during the morning rush hour.

Motorists should also expect delays on roads in the area.

The crash is the latest trouble for the Market-Frankford Line, which is SEPTA's busiest, carrying about 187,000 passengers a day.

Earlier this month, 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, which is a lateral beam that connects to the wheel assembly beneath. The cracks were caused by a faulty weld.

Tuesday's accident was unrelated to the bolster issue, Sauer said.

The crash, however, does further diminish SEPTA's El fleet, which had been on its way to recover from the earlier woes.

The reduced fleet had caused some disruptions on the Market-Frankford Line, but the transit agency had been again approaching full service on the line.

But now, the three six-car trains involved in the derailment will have to be inspected before they can return to service. SEPTA doesn't yet known how badly damaged those 18 cars are.

"It certainly doesn't help," Sauer said.