Isn't that mostly based on the fact that they share rail time? Its not like these tracks are unused in the other periods of time, they're just not Amtrak trains.
There used to be way way more lines forming more of a mesh than the skeleton we have today. Through a series of mega-mergers we are down to effectively a west-coast-duopoly and an east-coast-duopoly of companies that can run long-distance trains.
When railroad companies merge they tend to abandon one or the other's trackage in areas where the formerly-separate networks ran between the same market areas, keeping the minimum segments of track to cover the maximum number of customers (not even maximum geographical area).
