Compared with its ultramodern counterparts in Europe and Japan, Amtrak is not a font of innovation. But on its Heartland Flyer — a daily service between Oklahoma City and Fort Worth, Texas — Amtrak is taking tentative steps toward a greener, low-carbon future. Since spring, the Heartland Flyer has been running on 20% biodiesel rather than the carbon-heavy diesel fuel on which Amtrak's other trains — with the exception of the electric Acela Express — currently operate. The biodiesel reduces air pollution and helps cash-strapped Amtrak save on fuel. And appropriately for a train in cow country, the biodiesel is made from rendered cattle fat. Biodiesel from beef burns cleaner than plant biodiesel, though it may not be scalable outside the beef belt.