The Airtrain operated by the Port Authority takes you from Terminal to Terminal then to the Newark Train Station. From there your choice, Amtrak or New jersey Transit into Penn Station in New York City. We have used this numerous times in reverse, from Philadelphia area to the Newark-Liberty Airport. There are more traditional methods, however this is the most economical and not subject to traffic jams etc. http://tinyurl.com/k24a4

Vince & Gerry in Philadelphia