The appeal of Atlantic City International Airport is that it's isolated. It's not near any urban centers like PHL, EWR, JFK, etc. so going in and out is pretty easy. Newark Aiport is much closer to me, however, getting to Newark is a pain due to all the non-airport traffic to get there, plus the airport itself is far too congested to make getting in and out easy. I've used both airports before, and even including the drive to and from said airports, AC was always a faster trip overall.
I've never used PHL. If it's anything like EWR, I'd imagine a potential rail connection would expose more people to AC who would be willing to travel a bit further to an airport to spend less time waiting inside and outside it. However it doesn't sound like it's congested like the NYC area airports, so I guess an "overflow" airport like AC wouldn't be needed as much.
I'd imagine in a hypothetical scenario, if the entire Southern Division north of Winslow Junction was an active NJT route, you would probably have a lot of people from the Central/Southern Jersey population centers (Freehold, Lakewood, Toms River, etc.) take a train to the airport. However, that is not the case and probably never will be for the foreseeable future.
There have been talks to have some sort of industrial/research/office park (NextGen Technical Park and I think another one) near the airport that would attract people to use the airport, and perhaps even the railroad.