According to the 3rd Edition
According to the 3rd Edition of the American Shortline Railway Guide (1986), the Everett Railroad operated three different lines during its life. They started in 1954 operating "several miles of the abandoned Huntington & Broad Top Mountain Railroad near Everett, PA". That line closed in 1982 when Conrail discontinued service on the connecting line. The railroad therefore had no place to run from then until May 1984, when they acquired 7.9 miles of track from Sproul to Brookes Mills (both PA) from Conrail, then in 1985 they leased 6.8 miles of track between Roaring Springs, Martinsburg, and Corryville, PA that had previously been operated by the Allegheny Southern Railroad.
As noted, the creation of Conrail in 1976 really sparked a large number of shortlines formed to operate various branchlines and secondary main lines not included in the Conrail system. Many of these shortlines relied on public money to start up. Two seismic events in the early 1980s spawned the creation of many shortlines, specifically the abandonment of the Milwaukee Road system west of Miles City, Montana, and the subsequent sale of the rest of the railroad to the Soo Line in 1985, and the bankruptcy of the Rock Island. There were many shortlines that stepped forward to take over various parts of the Rock, mostly under "designated operator" appointments of the bankruptcy court. Perhaps one of the more interesting examples was the Cadillac & Lake City, which had been primarily a tourist railroad in Michigan, they moved west and for a number of years operated a former Rock Island line out of Limon, Colorado. Most of the railroads created out of the wreckage of the Milwaukee Road in the very early 1980s were typically associated with some of the larger shippers on the abandoned parts of the line to preserve rail service, such as Weyerhaeuser's Chehalis Western Railroad in western Washington (actually a resurrection of an older name) and Potlatch's St. Maries River Railroad in northern Idaho. The State of South Dakota purchased several hundred miles of Milwaukee trackage in that state and contracted operations out to several shortlines, including Dakota Southern, Dakota Rail, Sisseton Southern, and a few others (some of those companies operated the same line in succession.) Another major shortline startup in the west about that time was the Eureka Southern, which in 1984 purchased and started operating the northern end of the Northwestern Pacific, between Willits and Eureka, California.
There was another major wave of shortline creations in the last half of the 1980s. UP and CSX especially sold a lot of branchlines to shortline railroads in the 1986-1987 time frame, UP in particular identified and sold something like 86 branchlines in those two years. It should be noted some of these were bundled and sold as package deals. There was a major hiccup in shortline spinoffs in the roughly 1988-1989 time frame, the Pittsburg & Lake Erie Railroad was in dire financial straits and was attempting to sell itself in a way that would invalidate all union contracts, the unions took the railroad to court and won the first round or two. Since one of the biggest reasons for shortline sales was establishing the new railroads as non-union operations this decision had a chilling effect on further sales for a year or two, until the Supreme Court decided the issue in favor of the carrier.
In addition to the already referenced resources I'd suggest finding copies of the American Shortline Railway Guide, especially the 3rd edition already mentioned, the 4th Edition (1991), and the 5th Edition (1996). I think the 5th was the last. Both Trains and Railfan & Railroad have run many feature articles on shortlines through the years.
Hope this helps!
Jeff Moore
Elko, NV