True about the acceleration but the train is moving as fast as it is required to go once out of the station at 30th street. From the zoo (and the zoo is bordered by this Line) on out in a NE direction it is straight with a slight bend east at front street which is 30 blocks from 30th street. so if the banked curve is 50 the relatively straight speed could be much faster requiring a slow down at the curve which i don’t think that is how the trains are run there. I think they don’t go over 50 on that stretch.
I read on a train enthusiast site that has many members familiar with Amtrak in general and the area specifically that the limit was in fact 50 mph from Philly Station to the interconnect where the train derailed.
They also stated that the reason there was Positive Train Control on the south/westbound trains but not north/east was that it was considered inconceivable that anyone would get a train going that fast out of the station to reach derailment speed, while trains coming south are dropping from 125 mph limits to the restricted speed at the curve.