Are there any railway abandonments that proved to be regrettable in retrospect? This could either be as a loss of business for the railroad or as detrimental to the industry as a whole—any section of track where the railroad wouldn’t give up service, if it could be done over.
I'd say some of the lines CR and its predecessors downgraded and/or abandoned in the northeast are sorely missed. With all the congestion problems, another main line from, say Pittsburgh or Buffalo to Chicago or St Louis (like PRR's line) would be useful.
In New Jersey, the LHR's abandonment probably wasn't the best idea, as it could have been linked with other lines to create a bypass around the North Jersey terminal area. CR did study doing that, but I believe that the project's Achilles' Heel was that the Wallkill Valley (which would have linked the LHR with the river sub) was to light and badly maintained a RR to support large, heavy trains, and that trestle (Rosendale...?) was already falling apart (I think that it also was hard to directly link the two). In the scheme of things though, the LHR abandonment wasn't the end of the world; North Jersey isn't Chicago. I just like to think it is
In the west, one could argue that the MILW over pipestone pass shouldn't have been abandoned, as it was the newest and best-engineered of the northern crossings, but we wouldn't have any of the Milwaukee road left to work with if we hadn't let it die...
I bet I'm missing loads of others, but those are the ones I can think of off the top of my head.