I was aware of the "checkerboard" land-grant allocation, but not how deep (perpendicular distance from RoW) it went.
I also wasn't aware of how brief the land-grant subsidy era was. Though it would be interesting to see a track-mileage development timeline or animation showing how trackage developed over time.
My general understanding is that track mileage exploded quite quickly, often with competing railroads establishing duplicate connections especially on highly-attractive routes, with the winner then absorbing (or simply competing to death) the loser(s). There is also a distinction between major (national or regional) Class I railroads, and short-line (usually Class III) railroads, where a short-line often served only a single line or small set of lines within a small region.
Overall, US railroad trackage shrank for much of the 20th century as early, redundant, competitive trackage was abandoned. I'm not sure what recent trends have been.
However, it's also worth noting that land grants were only doled out in US history in an extremely limited time period--from the 1850s to the 1870s. The majority of railroads were not in fact built with land grants.