Newark airport's brand new rail station on the North East Corridor opens for business on September 30th, with both Amtrak and NJ Transit.
NJ Transit will have 3 trains per hour to and from the airport, and Amtrak will have hourly service. EWR- NY Penn Station is 20 minutes, on NJ Transit. Amtrak will link their Acela Regionals and Commuter trains with the Monorail at the NEC station. http://www.panynj.gov/airtrainnewark/travel.index.html
NJ Transit will serve the airport from NY Penn Station and it's NEC and North Jersey Coast line trains. Amtrak will serve the airport from as far North as Boston, as far South as Washington DC ,as far West as Harrisburg and points in between.