Nice story, but I'm skeptical. Why did the populace suddenly come to the conclusion that slavery was wrong when it had been practiced for tens of millennia?
As I've written in previous Pontifications, humans are nasty and brutish and some of them are short (or something like that). In addition, almost everything can be explained by power, and power alone. The history of slavery in America, in my opinion, is no exception.
I suspect that the heroes of the underground railroad were scorned much like PETA animal rights terrorists are today. They were likely considered lunatic extremists. Consider if in 50 years, animals actually do get full rights, then the PETA extremists of today will be remembered as visionary heroes!
So why then? What changed the power equation? First, consider that southern black slaves may have on average done better than their free northern counterparts:
Fogel and Engerman argue that slaves in the American South lived better than did many industrial workers in the North. Fogel based this analysis largely on plantation records and claimed that slaves worked less, were better fed [...] A survey of economic historians concludes that ... 23% "agreed" and 35% "agreed with provisos" with their argument that "the material (rather than psychological) conditions of the lives of slaves compared favorably with those of free industrial workers in the decades before the Civil War."Ultimately, I think that people are much better exploiting themselves than being exploited by others, and the free northern black, caring for himself, ultimately provided much better value per dollar which made the north more powerful. Even more importantly, rapid industrialization required flexibility of labor, and slavery is the most inflexible form of labor possible. The South was being held back economically by slavery. That is why it happened then.
Note that Fogel points out that the economics of agricultural based on slavery was more efficient than without it. Even if true, it doesn't change the fact that western civilization was in the midst of transitioning from an agrarian to industrial society. What mattered was not the efficiency of farms, but the efficiency of industry. The North, with only free men, excelled at industry and was therefore becoming ever more powerful relative to the South.
So then our morals evolved to catch up with the reality on the ground which was caused by power.
In the next pontification, I'll look at the fall of the European empires. The power equation changed, that's why the British left, not because they suddenly felt bad for the oppressed.