It has shut down a street, knocked down electrical wires and closed a trolley line and the bridge.
The tractor trailer tried to get under the bridge at Richmond and Somerset streets just after midnight on Tuesday.
When the big rig got halfway under the bridge it caught a girder which crushed the top of the trailer.
The truck could not move forward and could not back out.
Officials stopped all traffic on the bridge which is used by CSX freight trains.
They are inspecting the span to make sure it has not suffered any structural damage.
SEPTA has shut down the Route 15 trolley in the area, and is supplying shuttle buses for marooned passengers.
The truck is carrying a low level corrosive, but fortunately there aren't any leaks.
Crews are on the scene, trying to tow the truck out and repair the power lines.
Signs at the bridge indicate that it has only a 12-foot clearance.
The driver was not injured.
