I’ve always wondered if one of the reasons was to protect the Union. Would be a lot easier to prevent states from seceding, and defend US borders when you can quickly transport soldiers (plus equipment, food, etc.) to where they’re needed.
I don't think there is a single answer to "why the US built railroads", but both the railroads and the Eisenhower's Interstate Highway System certainly had something to do with the military needs.
I would imagine so. The CN railroad was specifically built to transport soldiers from eastern Canada to BC to defend against American machinations. (Not for expansion, nope, no way, we didn't want Alaska anyways!)