Union Pacific recently said it would spend more than $100 million to buy 20 battery-powered locomotives and charging systems from Wabtec and Caterpillar’s Progress Rail, the world’s biggest such purchase to date. Starting in 2023, the Omaha-based railroad will use the trains to pull cars around freight yards in California and Nebraska—rather than on cross-country runs—and estimates the mammoth machines could eliminate 8,000 tons of carbon emissions annually. Adding batteries is also a way to buttress efficiency: Trains haul freight over 480 miles per gallon of diesel fuel per ton. That’s up to four times better than the average for trucks, according to the Association of American Railroads.