^ Problem with that is how do you determine who has what trackage rights? Part of the reason several lines were successful in modern times (Union Pacific in bulk trains, Conrail with intermodals/stack trains and Norfolk Southern with coal hauling) is because they have the best routes. The best route for freight, especially if they are major systems as most of the class 1 railroads were, will all go nationwide on the best routes, which would result in horrendous traffic problems, which is counterproductive to better service.
Conrail, to be frank, was a necessary evil. Penn Central getting forced to take over New Haven just accelerated running PC into the ground. There was way too many lines and companies and simply not enough traffic. When PC went broke, the loss of car fees to other rail lines sank Reading Lines and Lehigh Valley within months if not weeks. I can see one way to dodge Conrail being the market-owning behemoth it became.
As the system is being set up in the early 1970s, build TWO railways. Penn Central stays in business but is allowed to dramatically rationalize its network. Much of the New Haven becomes the Amtrak Northeast Corridor. Boston and Maine and the pre-Guilford Northeast lines are also put into Conrail. A second company starts around the NY-Chicago main of the Erie Lackawanna, absorbs much of Reading Lines and Lehigh Valley, as well as the Boston and Maine, Ann Arbor and some outside portions of Penn Central.
The result is that Conrail and Penn Central are direct rivals across New England, Pennsylvania, Ohio, Indiana, Michigan and Illinois. If you want to drive the big lines apeshit, bring Milwaukee Road into Conrail and Penn Central, and have its Pacfic Extension operated by both. Burlington Northern would hate that, but it would force them to compete as the Milwaukee Road route is the shortest of the major Pacific NW routes by some 90 miles. The lines would both be transcontinental in that scenario.
The West is easy to sort out, as most of those didn't suffer the problems that the East did. BNSF was a merger of convenience, not necessity - both were successful lines and had major market shares, the merger was just done to create a more seamless system, and to compete with Union Pacific, which was the big kid on the block in the West.
Southern Pacific's problems were largely self-made. They were infamous for poor service, arrogant management and unfathomable stupidity, particularly in the California market. When BNSF happened, SP's terrible past and their now-dead-as-a-dodo market dominance forced a merger. President Clinton, in perhaps the biggest and most flagrant violation of NAFTA, scuttled an attempt by Canadian Pacific to buy Southern Pacific - which is little known but was a serious piss-off for CP - to the point they never really focused on US growth after that. You'd have to have a major improvement in SP's customer service, particularly in the Southwest, if they want to survive long-term.