Combining Union Pacific and Norfolk Southern into the first transcontinental railroad in the U.S. would create a 52,215-mile colossus that could offer seamless service from coast to coast, bypassing longtime interchange choke points in Chicago and at gateways along the Mississippi River.

Union Pacific said on July 29 that it had agreed to buy smaller rival Norfolk Southern for $85 billion, including debt, in a deal that would create the first transcontinental U.S. railroad.

The $85 billion deal announced on Tuesday would create the nation's first coast-to-coast freight rail operator and reshape the movement of goods from grains to autos across the U.S., which are issues ...