The Transcontinental Railroad was considered a major milestone for the United States for the following reasons:
- Businesses could transport goods across the entire country, expanding their reach.
- The completion of the railroad allowed for a much easier method for the shipment of goods.
- The completion allowed people to contribute and interact in places outside of their local economy.
The statement about the railroad leading to a decrease in automobile production as a means of transportation is not applicable, as the automobile era came later and was influenced by other factors. Therefore, the last option would not be considered a reason why the Transcontinental Railroad was a milestone.