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.
You can say the same thing about northern railroads west of the Appalachians; they are laid out east-west to get agricultural products to market and export. It’s just a lot more developed. Once again, the wealth supposedly created or held in the form of slaves does not translate into economic growth. Slavery is a bad business plan.
Interesting.
Well I do know that from a musical perspective - people think of the “Delta” from where “Delta Blues” derive as some old, hoary region - when in fact that opposite is true - growing cotton in the areas around the Mississippi from Memphis on down was a more recent arrival than most people think.
My point is that while the South looked like it had a lot of territory 1856 or what have you, it was as you say an agrarian nation whereas the North was rapidly industrializing. Railroads as a proxy for urban and industrial development *as well as* railroads as a legitimate instrument of modern war (that is to say useful in their own right for waging war and not just a proxy for industrial strength) tell a story different from the territorial one.