What term was used to describe the rough ride between free and slave states by the mid 1800s

The answer is:  Mason-Dixon Line

Mason-Dixon Line marked the northern limit where slavery was still allowed in united states (if you move one stop north from the mason-dixon line, you could be in jail if you owned a slave).

The line has not been moved for around 150 years before eventually the missouri compromise was made and reduce the area where slavery was still allowed.