What two poor races of people were exploited to help build the railroad lines quickly and cheaply?

They toiled through back-breaking labor during both frigid winters and blazing summers. Hundreds died from explosions, landslides, accidents and disease. And even though they made major contributions to the construction of the Transcontinental Railroad, these 15,000 to 20,000 Chinese immigrants have been largely ignored by history.

Explanation:

they died because of a disease during the first spanish colonies.