Pan Am Railways is actually a holding company for several railroad companies, including Maine Central, Boston & Maine, Springfield Terminal, and others. If considered as one railroad, it would be a Regional (Class II).
Their main lines make an “L” shape; the Fitchburg Division runs from Rotterdam Jct, NY (sort of near Albany) cross the state border into Massachusetts, then head east through the Hoosaic Tunnel, East Deerfield Yard, Fitchburg, and Ayer, and eventually reach Lowell. There are a couple branches off this route, including ones to Springfield and Worcester, MA.
Their Maine Central Division runs North from Lowell up to Bangor, ME, via Portland, and northward from there. This route has a whole bunch of branches. And finally their Boston & Maine Division runs up to Lowell from Boston on trackage rights, then goes up to Concord, NH.
Here’s a system map: http://www.panamrailways.com/Maps/map.htm
Their roster of around 100 (some of this number are currently out of service, so they probably have around 75-ish in service plus a bunch of HLCX leasers) is pretty neat. Built of mostly GP40s (not Dash-2 ones! [:)]), they also have a high-hood SD45, an SD26, and an SD39, among others.
Here’s a complete roster: http://www.thedieselshop.us/GRS.HTML
And here’s a Wikipedia page on the railroad:
http://en.wikipedia.org/wiki/Pan_Am_Railways
Hope this answers your questions!