My earlier posting on the Cowboy Line described what happened to it, but didn't discuss why it happened. I'll try to briefly describe that here. Keep in mind, though, that this is almost completely from memory, so dates are approximate and may be slightly off.
By the late 1970's, the Cowboy line was being kept alive primarily due to hope and mud. The "hope" (and plan) was that the line from Shawnee east would become part of CNW's planned route from the Powder River Basin. The "mud" was the bentonite clay moving from South Dakota south on the Colony Line to Dakota Junction (west of Chadron) and then either west to the UP or east to Fremont.
The hope and mud both vanished in the 1980's. The hope went first. Unable to obtain financing for its ambitious plan to build a new coal route which included the Cowboy Line across Nebraska, CNW turned to UP. The result was the existing UP coal route which only uses a small portion of the old Cowboy Line between Shawnee Jct and Crandall in Wyoming.
The mud went next, although its demise was a little more convoluted. CNW actually had two routes from the bentonite mining areas - the Colony Line / Cowboy route and a potentially alternate route via the Colony line and the line across South Dakota via Pierre (I say "potentially" because it wasn't being used for bentonite due to bridge issues). By the mid 1980's, CNW decided that it didn't need two through routes from Rapid City, and applied to abandon the portion of the South Dakota route between Pierre and Rapid City (the "PRC Line"), which would have severed it as a through route. Unfortunately for CNW (and the Cowboy Line), the ICC, at the time, had an even number of commissioners and, when the time came for them to vote on the abandonment, it was a tie vote. A tie vote results in denial.
CNW planned to reapply for the South Dakota abandonment, but fate stepped in. The prior abandonment proposal had galvanized the South Dakota political community, and they very actively pursued alternatives to the South Dakota abandonment. This eventually resulted in CNW selling not only the PRC line, but its entire line from Rapid City to Winona MN to a new regional road - DM&E. As part of this deal, DM&E agreed to address the bridge issues that had prevented CNW from routing bentonite over the line (I believe they got federal funds to do this, but I'm not sure). CNW, in turn, agreed to reroute the eastbound bentonite traffic over the South Dakota line once the bridge work was done, which is what happened.
This pretty much sealed the fate of the Cowboy Line between Chadron and Norfolk (over 300 miles), as there wasn't enough local traffic generated on the line to sustain it (it was primarily agricultural traffic most of which had been lost to trucks). ICC approved "discontinuance" of service over the route in 1991 (effectively an abandonment where he track stays in place). ICC approved full abandonment of the segment between Norfolk and Merriman in 1994, and the sale of the Merriman-Chadron segment to NEBKOTA (a short line). As mentioned in my prior post, NEBKOTA has since received abandonment authority to abandon most of this segment.
Following the abandonment of the line segments east of Crandall, the Casper Line became essentially an island, dependent on a dwindling local traffic base. CNW obtained trackage rights over BN between Orin and Casper in 1989 or so, which allowed it to abandon most of the Casper Line. UP abandoned the remainder of the line and the BN trackage rights after the UP-CNW merger (it had been on CNW's chopping block, but the merger occurred first).
Clear as mud?