The Market-Frankford Line train. (Emma Lee/WHYY)

A train on the Market-Frankford Line partially derailed late Saturday night as it traveled north out of Old City, transportation officials said.

No injuries were reported, according to SEPTA spokesperson Andrew Busch, who said passengers were evacuated to shuttle buses. The incident happened shortly before midnight, and service was disrupted while transit crews sought to discover what went wrong. 

Working overnight on a fix, crews found and repaired a crack in one of the rails, Busch said. Service, which regularly stops between 1 and 5 a.m., was a few hours late in restarting Sunday morning, with trains running by just after 10 o’clock.

Small derailments happen relatively often on heavy rail like Amtrak, to the tune of more than 1,000 a year nationwide, though we usually only hear about them when there’s extensive damage or injury. It happens on Regional Rail too; a SEPTA train derailed in Trenton last fall.

But it’s rare on light rail like the MFL. Over the past decade, SEPTA tracks in Philadelphia only saw five derailments, according to Federal Railroad Administration data.

The cause of what happened in Old City still hasn’t been determined, per Busch, and the transit authority has notified the NTSB.

Here’s a look at what else we know about the incident.

Where exactly did the derailment happen?

Between 2nd Street Station and Spring Garden Station. That’s right where the route curves northward on its way to the Frankford Transportation Center, and passes under the Ben Franklin Bridge.

What kind of train derailed?

It was a standard six-car train, per Busch, and the third car is the one that somehow slipped off the track.

Many of the train cars that traverse the El have in the past needed repairs. They’re a design class known as M4, and have been in use for about 25 years. SEPTA is currently bidding out a contract to produce a fleet of M5-class cars for the MFL.

How many people were on board?

About 100 passengers.

SEPTA maintains there were no injuries, though a Fox29 reporter tweeted that one woman was taken to the Hospital of the University of Pennsylvania with complaints about hurt knees.

Shuttle buses were substituted to run between 5th Street and Huntingdon Station, frustrating some late-night passengers.

What happened to the tracks?

A crack “in a piece of rail” was repaired overnight, Busch said, “but we still have more work to do to determine exactly what caused this.”

Extremely cold temps like Philly saw this weekend can cause train tracks to become brittle or shrink, and be more prone to cracks. (Chicago’s Metra sometimes lights its rails on fire in winter to help them expand and anneal.)

Asked if the weather had anything to do with the Old City incident, Busch said the authority didn’t yet know.

What about the train car?

The train itself is now in the rail yard, Busch said, and is being examined for any abnormalities or clues as to why the derailment happened.