We do regulate interstate commerce in America for a variety of reasons, including to guarantee consumer sovereignty by monopolies, or monopolistic practices, and that includes most notably, railroads.

Railroads may be a natural monopoly for geographic, historic, or economic reasons, and if they are found to be engaging in practices that are monopolistic we can regulate them accordingly to eliminate such practices. Geographic and historic RR monopolies are self explanatory, and the primary economic reason for economic RR monopolies is the "barriers to market entry", in this case, the exorbitant cost to construct a competing RR line to serve the "captive" market, leaving existing "monopolist" carrier dominant to treat customers however they want.

Open access, or mandatory access may be warranted, if the carriers monopolistic practices do harm to other economic actors. And, I seen such cases right here is CA, and how such a monopoly can do significant damage to customers and shippers, like with what has happened at Port Hueneme, where open access might be one means of preventing that kind of monopolistic behavior.

A priori comments on how this would "slow down" or otherwise impact existing carrier RR operations are premature. This is not really known, and would vary considerably, depending on the route, traffic levels, operating capacity, track time and traffic to be served. There are also clearly many places where this argument is clearly just an excuse, not an actual problem.....