The railroad map is awesome. But it does shed a different light than the original map. The original map (to me) creates an impression of slavery on the ascendancy. The railroad map shows that the slaves were where the railroads were not as dense and vice versa.
If you study the Southern railroads carefully, you see that they are focused on bringing product -- mainly cotton -- to harbors for export worldwide.
Their starting points are those areas of highest production.
As for imagining slavery in areas of no railroads, no don't do that, because it was just the opposite.
What you should understand is that even by 1860, the Cotton South had only begun to use about a third of the land suitable for cotton.
By 1900, acreage producing cotton was triple what it had been in 1860.
So, where there were no railroads in 1860, there were very few people, period.
That's how some people say, on average, Southerners lived closer to a railroad than their northern cousins did.