When the construction of the transcontinental railroad was completed,

The transcontinental railroad connected the eastern and western parts of the United States, making cross-country journeys much faster than travel by wagon or stagecoach. Therefore, the best answer is that it made travel time shorter between the East and West. It also carried goods, but the question asks about the broad effect of its completion, and faster travel was a major result.

AI-written and checked against the verified answer.