The PRR main was never completely four-tracked, but it held something close to that status from the West end of the Station and freight yrds at Newark/Jersey City, NJ to a point at or near Rochester, Penna, about 25 miles west of Pittsburgh.
Between Trenton and Harrisburg, the definition of the line's status was complicated by the "Trenton Cutoff", a freight-oriented line enginerred primarily with the object of minimum grades that diverged from the passnger main in Morrisville, Penna, on the other side of the Delaware, skirted Philadelphia to the north, (with much trackage paralell to the Pennsylvania Turnpike after that highway's completion) re-crossed the old Main Line near Parkesburg, then crossed the Susquehanna at Columbia, about fifteen miles south of the capital city. It then paralelled the Susquehanna to the Enola freight yards and the west end of the Rockville Bridge.
The St Louis, or Southwesten line diverged from the passenger main at Pittsburgh, but most east/west traffic continued on the Chicago line after the development of Conway Yard, near Ambridge, as the Pennsy's major freight-classification hub in the 1950's. Freight for the southwest then returned to the former "Panhandle" route via the River Branch and Mingo Junction, near Steubenville. Once that line and the line to Cleveland diverged, two tracks were generally sufficient for traffic to Chicago, and until the completion of Conway, much of the freight portion of that operated from Pitcairn and Scully yards on the Steel City's outskirts, via Columbus and Logansport, Ind. As passenger traffic dwindled aftwr World War II, more Chicago-originated freight found its way onto the "Fort Wayne Line".
In addition, the Pennsy's secondary lines, including the Conemaugh, Williamsport/Buffalo, Bald Eagle, Allegheny and H&P/New Portage, were all arranged so that a major blockage due to a wreck culd be bypassed. Most interesting of all, in this writer's opinion, was the Low Grade Branch, which diverged from the Buffalo line at Driftwoood, PA, about 20 miles north of Renovo, and continued west via DuBois, to rejoin the Allegheny Branch at Brady's Bend. At one time, plans reportedly called for this line, which was the lowest crossing of the Eastern Continental Divide except for the NYC main, and required no helpers, to continue westward into Ohio and the lake ports, a paralell of sorts to US Route 422 and later, Interstate 80. It's interesting to speculate on what might have been if this route had been developed more agressively, or used in place of the old Main Line-Erie via Warren, Penna.
What a revoltin' development this is! (William Bendix)