I was there last week. For the return trip home, instead of taking a cab, I opted to keep project expenses down and go the train route. I grabbed a train schedule, made my plans, and arrived at the airport by train in plenty of time for my Delta flight.
Notice the area in the red circle on the train schedule below: it shows that Delta is at Terminal A of the Philadelphia airport.
Philadelphia Train Schedule
When I arrived at Terminal A, I had no success in locating the Delta counter. I finally broke down and asked a security guard for help.
"Delta? That's Terminal E."
"OK. The train schedule said it was Terminal A."
"Delta has been at Terminal E for nine months now."
"Thank you," I said, and went in search of a shuttle to the other terminal.
Good thing I made it to the airport in plenty of time.