I recently ran across a mention that this was a deliberate decision made in the 1950s to help further break the power of railroad monopolies.
The decision had merits at the time. Side-effects may be overwhelming those now, though I'll note that with only five US majors, centralisation within rail remains a concern.
A better way to do that would have been something akin to the "Rail Packages" of the EU, most notably the open access part. Infrastructure should be separate from operations, and access should be given to anyone at equal priority and fees. Monopolies are effectively broken, and that works for rail as well any other type of infrastructure monopoly - ISPs, power, etc.
The decision had merits at the time. Side-effects may be overwhelming those now, though I'll note that with only five US majors, centralisation within rail remains a concern.