This study primarily establishes two things: (1) that monopoly has been pervasive in the U.S. water transportation industry in both the 19th and 20th centuries and has led to prices above competitive levels and the adoption of inefficient technologies and (2) that the competition of railroads has greatly weakened this monopolistic tendency, leading to lower water transport prices and fewer inefficient technologies. The study establishes these points using standard economic theory and extensive historical U.S. data on the behavior of unions and shipping companies. These gains from competition have been ignored by researchers studying the contribution of railroads to U.S. economic growth. Researchers have assumed that if railroads had not been developed, the long-distance transportation industry would have been competitive. This study shows that it would not have been. The quantitative estimates of previous studies thus are likely to have significantly understated the gains from the development of railroads.