Chief Troll makes a good point about EL rail. This factor is almost always overlooked by historians of EL's demise.
From the late Fifties onward, Erie, DL&W, and EL were seldom able to put together the cash to undertake large-scale rail projects, especially in view of the urgent need for other more urgent capital expenditures. EL narrowly avoided bankruptcy in 1963 and was in trouble again by 1969.
As a result, while NYC and PC had been steadily converting to continuous welded rail (CWR), EL's main track still included a lot of 112, 115, 130, and 131 lb. stick rail. On the "old main line" via Wellsville, eleven miles of 1920s 110 lb. rail remained. Some of the 112 and all of the 110 and 130 was rolled before control cooling of rail was required, meaning that it probably exhibited higher rates of internal defects. (I believe that at least some of the 127 lb. rail that was the NYC's mainline standard, which was cropped and welded all across the system, was also non-control-cooled.)
Much of the time, the 131 could have been cropped and welded for continued use on EL's main line. Likewise, control-cooled 112 and 115 lb rail was suitable for re-use as either CWR or stick rail on branch lines and in main line passing sidings.
Occasionally, it is alleged that management was slow to undertake the single-tracking projects that might have permitting concentrating limited resources on fewer tracks. Certainly, by the early 1970s EL was still attempting to maintain way too many miles of double track. However, this criticism overlooks the fact that single-tracking projects required up-front capital investments in signals and switches, if they were done right. The hatchet job that was done on the Delaware Division in 1973 showed the harm that could result when it was done on the cheap.
Another little factor is that the 131 and heavier rail used a different size tie plate than the 110, 112 and 115 rail, meaning that the spike holes fell in different places.
In hindsight, EL's installation of CWR in Western New York and Pennsylvania seems carefully calibrated to the gross tonnage over each line segment. Perhaps this simply reflected head wear and curve wear at the time. Example: The Buffalo Division westbound track between Hornell and River Junction, which saw most Allegany Division freight traffic in addition to Buffalo trains. Or the single track segment of the main line between RH (west of Steamburg) and Waterboro. Or the Columbus & Erie RR from about MP 7 to Columbus, which functioned as the eastward main line and I believe also handled some westbound traffic by the 1970s. Or the single-track line west of Meadville. Many short segments of CWR apparently were necessitated by excessive head wear or curve wear of the predecessor stick rail.
A study of the available data indicates that Erie Lackawanna's installation of CWR in western New York and northwestern PA occurred predominantly in 1966--when EL had a little cash and a supply of heavy rail from the DL&W--and in 1974-75, when the company was spending government loan funds.
Most of the time, the presence of CWR in the track played a major role in Conrail's single-tracking decisions, although bridges, sidetracks, and other were also big factors. In the late '70s and early '80s, Conrail welded rail program was hungry for reusable CWR and stick rail that could serve as feedstock (no different from EL's treatment of DL&W lines in WNY). When studied up close, however, Conrail's single-tracking of the former EL main line usually left the existing CWR in place.
Regarding the larger question of what-lines-did-Conrail-keep-and-why: Erie Lackawanna, which had historically functioned as NYC's competitive shadow, was a stepchild within Conrail. The ex-EL lines were redundant only because they were owned by Conrail. Had they been owned by Chessie or N&W, or by MARC-EL, they would have been core main lines and presumably would have been rebuilt with CWR, just as Conrail did to the still-significant mileage of ex-NYC and PRR main lines that remained stick rail in 1976.
Toward the end, there is no question that Conrail sought to abandon a large segment of the former EL between Hornell and Meadville in order to prevent a potential connection between CSXT and D&H. This is not speculation; it was stated by David LeVan in a 1995 meeting with Congressman Amo Houghton, in which I was present. That policy was echoed many times by his subordinates, who understood well enough that defending the monopoly was always the prime directive.
WDB