"Some of the sites were very difficult to locate because there were no landmarks in the photos," says Li, who is also a computer engineer.

"History and geography are closely connected. In another 150 years, with the changes in landscapes, it will be impossible to identify those sites," he says.

Li says he plans to donate his photos to the Chinese Railroad Workers in North America Project at Stanford University, which aims to create an online archive and digital visualizations of the era.

Two photo exhibitions featuring Li's images paired with Hart's photos are currently on display at the Utah Cultural Celebration Center in West Valley City and the Museum of the San Ramon Valley in Danville, California. Both exhibitions are sponsored by the Stanford project.

"The railroads connected commerce between the west and the east. People should know who built the railroads-it was the Chinese workers," says Paul Fong, a professor of political science at the Evergreen Valley College in San Jose and a former member of the California Assembly.

The First Transcontinental Railroad, originally known as the Pacific Railroad, was a 3,069-kilometer-long continuous track completed on May 10, 1869, linking the Pacific west coast with the Atlantic east coast for the first time in US history.