■ From the Field

Warren Buffett did not buy Union Pacific. He bought Burlington Northern Santa Fe — and the distinction, long treated as a footnote, turns out to be the whole story. This week's piece examines what it means that BNSF's profitability is now being measured against the very railroad Buffett deliberately declined: the financial instrument becomes the yardstick for the franchise. It is a structural question with a decade-long answer, and the field is watching it resolve in real time.