Yes, only one track was built, and the majority of the route from Omaha to Sacramento remains single-track today.

Only a few trains each day were expected on most of the line, and double-tracking would require twice as much work. The two companies were racing against time and creditors to build as much mileage as possible, because they were being paid by the mile, not by traffic capacity. Passing sidings were placed at strategic intervals, and busy sections had a second track added later. By the time of the transcontinental railroad, the telegraph allowed for coordinated movement between stations. That wasn’t yet in use when some of the first heavy-traffic British railways were built, and double-tracking was there considered the more practical solution to running trains in both directions.

I can’t think of any American railroad—except for short stretches entering major cities—that was originally built with multiple tracks. Only about 11 percent of the US railroad network is double-tracked.