Train 9741 en route to Trenton lost power and was stopped in the area of 34th Street and Girard Avenue just before 5 p.m.
Passengers complained of being stuck inside the hot train cars for hours, with no air conditioning and temperatures outside approaching 100 degrees.
"I started to feel claustrophobic because I wanted to get off the train, and actually couldn't do anything because we were stuck," said passenger Tiffany Ledesma.
SEPTA said the train's pantograph, a device on top of the train that brings in power from an overhead wire, lowered unexpectedly forcing the train to lose power. Attempts to troubleshoot were not successful.
"It appears to be heat related," said SEPTA Assistant General Manager Scott Sauer. "When we get into this type of temperature this can happen because the wire begins to sag in certain locations and that's where the heat takes a toll on the wire."
Almost 350 passengers eventually unloaded from the train cars.
A second train designated to get them had a medical emergency on board, so they were forced to wait in the heat for a third train.
"The trains took an extremely long time to pick us up," said passenger Richard LaBoy.
One passenger said their normally 30-minute commute took approximately 2.5 hours due to the incident.
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