I agree with MSP2000.

I hear the bus from Chinatown (in "center city" [=downtown] PHL) is pretty cheap and decent.

If used to take the SEPTA R7 to Trenton and NJT train to NY Penn Station when I lived in PHL. Not only cheaper than Amtrak (a bit longer but not much), but if you are going over the Labor Day weekend, Amtrak may be booked and/or jammed.