Land value underneath railroad tracks is an interesting subject. Most land value is reasonably calculated by width * length, and maybe some airspace rights. And that makes sense to our human brains, because we can look at a parcel of land and acknowledge it might be worth $10^x for some x given inflation.

But railroads kind of fail with this because you might have a landowner who prices the edge of their parcel at $1,000,000,000,000 because they know you need that exact piece of land for your railroad, and if the railroad is super long you might run into 10 of these maniacs.

Meanwhile the vast majority of your line might be worth less than any adjacent farmland, square foot by square foot, especially if it’s rocky or unstable etc.

Having a continuous line of land for many miles also has its own intrinsic value, much more than owning any particular segment (especially as it allows you to build a railroad hah).

Anyway, suffice to say, I don’t think “land value underneath railroads from the 18th century” is something that’s easily estimated.