I would make a very simple change. Instead of net book value, I would have net salvage value, which is exactly what the CTA is using today in the case of selling a line.
If you're not careful, it would be advantageous for a railroad to sell the line at net salvage value and get whatever the agency determines that way, as opposed to receiving nothing under net book value. The net book value is not the right standard, not for assets that have historical values that were set at the beginning of the century.