The growth of railroads from 1850s to 1880s made commercial farming much more feasible and profitable. Millions of acres were opened to settlement once the railroad was nearby, and provided a long-distance outlet for wheat, cattle and hogs that reached all the way to Europe. Rural America became one giant market, as wholesalers bought the consumer products produced by the factories in the East, and shipped them to local merchants in small stores nationwide. Shipping live animals was slow and expensive. It was more efficient to slaughter them in major packing centers such as Chicago, Kansas City, St. Louis, Milwaukee, and Cincinnati, and then ship dressed meat out in refrigerated freight cars. The cars were cooled by slabs of ice that had been harvested from the northern lakes in wintertime, and stored for summer and fall usage. Chicago, the main railroad center, benefited enormously, with Kansas City a distant second. Historian William Cronon concludes: