If trading opportunities were the answer it would have been reported as much at the time imo. Here is the justification from the U.S. house of representatives about the project at the time, in 1856:

"The necessity that now exists for constructing lines of railroad and telegraphic communication between the Atlantic and Pacific coasts of this continent is no longer a question for argument; it is conceded by every one. In order to maintain our present position on the Pacific, we must have some more speedy and direct means of intercourse than is at present afforded by the route through the possessions of a foreign power" (1)

The reason seemed to be just geopolitics rather than trading opportunities. I'm curious if you know what route they were referring to in this quote that routed through a foreign power? An oversea route maybe? In 1856 we had territory coast to coast already so the caravan routes were within our possessions.

1. https://en.wikipedia.org/wiki/First_transcontinental_railroa...