Whomever happened to be the latest bunch ...
the most famous were the Chinese and the Irish.
Irish
Chinese did most of the work from the west coast with Irish labour employed from the east
The Chinese
Most of the labour used to build the Canadian Pacific Railway were immigrants because they would work for less pay. On the eastern side the largest ethnic group would have been Irish. On the western side it was the Chinese.
Any group that was easily available in quantity and would do very dangerous work for almost no pay.
Chinese immigrants were the most prominent laborers.
The minorities
Both the Irish and the Chinese immigrants helped to build the railroads in America.
The Chinese were asked to help build the Canadian Pacific Railroad. Many original tracks that dodged the mountains have been relayed to make a safer and newer route.
Chinese immigrants played a significant role in building the Canadian Pacific Railroad. They were recruited to work on the construction of the railway in the late 19th century, facing harsh working conditions and discrimination during the process.
Australian Railroad Group was created in 2000.
The largest immigrant group in the US are Mexicans, of which about 800,000 Mexicans migrate to the US every year.