I don't remember the book, but they discussed how when congress was holding hearings after the Penn Central debacle Staggers was one of the Senators on the Committee. I think he was questioning Al Perlman about what went wrong and Perlman gave him both barrels. Essentially laid out how the Feds were as responsible as the RR management for about 50 years of bad policy that created the conditions for the Penn Central failing.
It's hard to point out a single factor, but I agree had deregulation happened earlier, the railroad business would likely be more dynamic today and a lot of railroad mileage (and jobs) would probably still be around, even if they'd of ended up a shorelines or regionals. Even when the Rock folded the bankruptcy court diddled around until some lines weren't worth reactivating (like the ATSF wanting Amarillo to Memphis or the D&RGW wanting Denver to KCMO & Council Bluffs.)