Caucasian and oriental were the two main ethnic groups. The labor force was made up Chinese and Irish immigrants, as well as veterans of the American Civil War. The veterans would have included African Americans and Hispanics. Source: Wikipedia, First Transcontinental Railroad.
If speaking purely on ethnic groups responsible in building the American railroad system -- the answers are: German, Irish, and Chinese.
Chinese and Irish
The western portion of the First Transcontinental Railroad, called the Central Pacific, was built mostly by Chinese immigrants. The popular term for these laborers was Celestials since China was known as the Celestial Kingdom.
ChineseApex
The Chinese
The Chinese
American Indians and the Chinese
They were escaping from the harrasment and discrimination they experienced.
Population Transfer and Genocide
Chinese
The country in which President Clinton employed the use of force to end ethnic cleansing was Serbia.
Mohd. Azizuddin Mohd. Sani has written: 'Dynamic of ethnic relations in Southeast Asia' -- subject(s): Politics and government, Cultural pluralism, Ethnic conflict, Nation-building, Ethnic relations