An out-of-service SEPTA Market Frankford Line subway train crashed into two other trains at the 69th Street Terminal just outside Philadelphia Tuesday morning, injuring four people and knocking seven cars off the track during the busy rush-hour commute.
Tuesday night officials announced trains would be in service at the terminal Wednesday morning.
"It's still undecided whether or not they will use the loop first thing in the morning," said Scott Sauer, SEPTA's Assistant General Manager of System Safety.
The collision left the operator of the No. 57 train critically injured, the Southeastern Pennsylvania Transportation Authority said. Another operator and two passengers were also injured in the crash but the injuries did not appear life-threatening.
It was not clear why the passengers were on the train since it wasn't in service, said SEPTA spokesman Andrew Busch. Busch didn't have details on their injuries but said they have been described as non-life threatening. The second train operator was treated and released, he said.
Upper Darby police asked commuters in a tweet to avoid the busy terminal after the three-train wreck on looping turnaround tracks, where trains turn around at the end of the line.
Investigators said the No. 57 train slammed into the back of the No. 67 train -- both trains were waiting to make the return trip to Philadelphia -- and the wreck then sideswiped the No. 51 train traveling in the opposite direction on another track. The trains were out of service at the time of the crash, SEPTA said.
SEPTA said seven cars were derailed. SkyForce10 footage showed one car tipped over at a 45-degree angle -- its wheels dislodged from the body of the train -- and six other cars partially off the track as crews responded.
A man who lives near the scene told NBC10 Philadelphia's Pamela Osborne he heard a loud noise followed by the sound of fire engine sirens.
"I heard a big bang...I knew something big happened but I didn't know what until I got here and saw this mess," William Stamm said.
The wreck left the 69th Street stop out of service for hours as state officials and National Transportation Safety Board investigators investigate the crash, SEPTA said.
SEPTA used shuttle buses to get passengers from 69th Street to 63rd Street. Passengers could be seen boarding the buses around 9 a.m. The line that runs from Upper Darby to the Frankford section of Northeast Philadelphia experienced delays throughout the day.
The crash impacted West Chester Pike, Market Street and Victory Avenue at one point, police said.
SEPTA got trains moving again early Tuesday afternoon. The trains operated out and back into the terminal as the loop remained closed so investigators could sort through the scene. SEPTA riders experienced delays and crowded conditions during the evening rush.
The Market-Frankford Line is equipped with advanced signaling technology called Automatic Train Control, or ATC, which should prevent two moving trains from the same section of tracks, former SEPTA spokesman, and current NBC10 employee, Manny Smith said. A SEPTA headquarters dispatcher would also be controlling the line and giving permission to engineers to move into and out of the loop.
The systems in place ensure optimal turnaround times at the terminal since trains at peak hours arrive at least every four minutes, Smith said.
The max speed on the curve is 10 mph, SEPTA said.
The MFL Line has been operating with limited cars due to under-body crack concerns.
This crash comes nearly two years after a deadly Amtrak derailment in Philadelphia's Frankford neighborhood.