If that were the case, the Emancipation Proclamation would have freed all slaves, not just those in the 'South'. In fact, the 'North' had slaves before the 'South', and kept them after those in the 'South' were free.
It is not surprising, therefore, that the 'underground railroad' did not end until it reached Canada. There was no freedom for slaves in the 'North'.
It remains fashionable to paint the US, and especially the south, as the prototypical bastion of slavery. Being so passionately fashionable wrongly convinces the rest of the world the we are the worst slavers of all time. In fact, quite the opposite is true.
Perhaps you need to look up "tactics".
"It is not surprising, therefore, that the 'underground railroad' did not end until it reached Canada. There was no freedom for slaves in the 'North'." Surely you are just pretending to be this dense. Or could it really be that you never heard of "The Fugitive Slave Law"?
This abomination made all Blacks suspect and unsafe in the North. Only in Canada were they safe. This does not mean that Northerners liked Blacks much most didn't. But only Republicans believed there was no right to enslave them.
Nor is it "fashionable" to paint the US as a whole as slave holding but it is true that a tiny minority of Southerners brought the disaster of war upon the whole. Defenders of Slaverocracy DO try to paint the US as if it were as bad as the South but not defenders of freedom.