The Underground railroad was a major cause of friction between the North and South in the United States. Many northerners sympathized with those who helped bring slaves to safety. Southerners for many years pushed for strong laws that would force the reacpture of escaped slaves, and in 1850 Congress passed a law mandating the capture of fugitive slaves. This prevented slaves from settling in free states and forced them to escape to Canada.
The main destination of the escapees was southern Ontario around the Niagara peninsula and Windsor, Ontario. About 30 000 individuals successfully escaped to Canada. This was an important population increase to the still underpopulated Canadian colonies and these settlers formed the basis of the Black population throughout Ontario.
For underground railroads and railways in the general sense (subways, metros), see metro.