Final answer:
Chinese and Irish immigrants were the main labor forces behind the first transcontinental railroad in the US.
Explanation:
Chinese immigrants and Irish immigrants were the two major immigrant groups that provided large numbers of laborers for the construction of the first transcontinental railroad in the United States.
Chinese immigrants were heavily recruited by the railroad companies, with many working on the Central Pacific section of the railroad, particularly in challenging terrains like the Sierra Nevada mountain range.
Irish immigrants were also employed in significant numbers, especially in the early nineteenth century, contributing to the labor force that built the railroad tracks across the nation.
Learn more about Transcontinental railroad construction
