What was the major milestone in transportation in the American West?

The major milestone in transportation in the American West was the First Transcontinental Railroad.

Explanation:

The First Transcontinental Railroad was built in the 1860s and connected the central and eastern states of the country with the California coast of the Pacific Ocean.

During the Civil War, railways played an important role for the two conflicting parties, who used rail to deliver weapons, food, ammunition, and troop movements. Therefore, in 1862, President Abraham Lincoln signed the Pacific Railroad Act that authorized the construction of the Transcontinental Railroad, which was supposed to connect California with the eastern states.

The railroad instantly changed the demography and economy of the American West. From the east of the country, English-speaking migrants came to develop new territories, crowding out the indigenous Indians and Hispanic Creoles in California. By the beginning of the 20th century, in different directions from the first road and parallel to it, new, more modern transport systems were built, which are still operating.

rail roads we the major step in transportation.