thinking about it some more, the key is actually can each of those (BE and BNSF) outspend their tax depreciation, not their accounting depreciation....I think.
Anyways, I know why the difference exists, but am not rigorous enough to project how that will change over time, because I more or less choose to value those subs on comps at market value in base case NAV and haircut them in the bear case NAV.