Compared to other parts of the world, US railroads began in the mid-late 1800's as small private companies with little incentive to cooperate. In fact, most were vigorous competitors in their geographic area.
Without national government control and financial support, private RR companies didn't have the financial ability to build a wholly owned coast-to-coast rail network.
In fact, during WWI the US Federal Government nationalized the RRs to get them to cooperate better. But things went back to the old competitive ways after the war.
Finally, there was the Western RR consortium vs the Eastern RR consortium, which had an uneasy truce in places like St.Louis, Chicago, and New Orleans. In those locations, jointly owned terminal RRs were created so transcontinental shipping could be reliably done between east and west coast RRs.
Chris
"True rail fans have two favorite railroads. The B&O and one other."