Web forums with specific sections for the NYC, PRR, PC and CR that I am involved with are:
http://www.trainboard.com/
http://forums.railfan.net/
http://www.railroadforums.com/

Additionally, there are e-mail discussion lists (forums) that are dedicated to only NYC, PRR, PC or CR. In more or less alphabetical order, they are:

Conrail-Talk:
A discussion list dedicated to sharing information about Conrail with some allowances for predicessor and successor roads.
To subscribe, e-mail
listserv@dsop.com
Subject: subscribe Conrail-Talk
Message: subscribe Conrail-Talk Your Name Here

NYCmodeler:
A discussion group for anyone interested in modeling New York Central System and subsidiary lines. Topics need to relate to modeling of the NYC, affliated and predecessor railroads.
To subscribe, e-mail
NYCmodeler-subscribe@yahoogroups.com
Post message: NYCmodeler@yahoogroups.com

NYC RR:
A discussion group for friends and former employees of the "Water Level Route", the New York Central Railroad Company. The focus is primarily historical, and mostly limited to the NYC (and PC successor) period prior to ConRail.
To subscribe, e-mail
NYC-RR-subscribe@yahoogroups.com
Post message: NYC-RR@yahoogroups.com

NYC-Railroad:
A place to discuss the New York Central Railroad's US operations, including the B&A, P&LE, P&E, among others, as well as its Canadian divisions and branch to Ottawa. Topics are welcome on anything from the earliest days to the current day operations on the ex NYC lines.
To subscribe, e-mail
NYC-Railroad-subscribe@yahoogroups.com
Post message: NYC-Railroad@yahoogroups.com

PRR-Talk:
Dedicated to discussion of all aspects of the 'Standard Railroad of the World', routes, equipment and modeling. To subscribe, e-mail
listserv@dsop.com
Subject: subscribe PRR-Talk
Message subscribe PRR-Talk Your Name Here

penn-central:
A Penn Central Railroad mailing list. This mailing list is for discussing all aspects of the Penn Central Railroad, including but not limited to: operations, motive power, track arrangements, predecessors, etc.
To subscribe, e-mail
majordomo@smellycat.com
Subject: subscribe penn-central
Message: subscribe penn-central
Note: you do NOT include your name.

PCModeler:
A list for discussion about scale modeling of the Penn Central Railroad in miniature. Subjects about predecessor railroads (Pennsylvania and New York Central) or subsidaries are welcome as they relate to the PC.
To subscribe, e-mail
PCModeler-subscribe@yahoogroups.com
Post message: PCModeler@yahoogroups.com

Note: For the Yahoo lists, you can sign up on the web at Yahoo. It will be necessary to sign on to Yahoo to use the web facilities of those lists.