Why did the underground railroad not stop in united states but run all the way to canada?

The Underground Railroad ran all the way to Canada, because of The Fugitive Slave Act passed in 1850. This Act allowed slave owners to come to the North, and take slaves (and some African Americans that weren’t slaves) back to the South. To avoid this happening, slaves would go all the way to Canada, since Canada didn’t have this law, resulting in the Underground Railroad going all the way to Canada.

Both countries had to aprove annexation.