The show in the Lehigh Valley is almost entirely Norfolk Southern, although R J Corman does maintain a repair facility there.
Probably the best place to start is the Allentown classification yard; the hump is readily visible from a series of local roads which paralell the north side of the river.
From I-78, take US 22 (the older expressway on the north side of town) to I-378 (downtown distributor) to Exit 2 (Eighth Avenue). Turn right onto Eighth, which will end at the river. The yard is further west (turn right).
The former Jersey Central mainline to Jim Thorpe and Scranton sees only 4-8 daily moves, and 2-4 of them are locals. Most of the action is on the Reading Line, which gets about 35 daily freights, with a modest peak between 6PM-2AM.
Between Macungie (about 8 miles to the west on Penna Route 100) and Reading, the NS main is almost entirely viewable from local roads. There was some discussion of this last summer, and I've posted a link.
http://www.railroad.net/forums/viewtopi ... ht=#450944