The diesel locomotive is the dominate type of train used world wide. A few steam locomotives are still operating for tourists but the diesel engine is the work horse and subsequent backbone of the railroad industry.
Earlier types of diesel trains pulled multiple types of freight cars including boxcars, flatcars, tank cars, reefer cars, open and closed hopper cars. Today the diesel train is more specialized pulling mainly one type of freight car. If you live in the great plains you'll see trains made up of coal cars both full and empty running from the mining companies to utility operations. These diesel trains pull over a hundred coal cars daily; in fact if you live near a mainline you will more than likely see at least one diesel coal train every single day, coming and going. Of course you may get somewhat irritated by the sound of the diesel horn as the train rolls through town.
In the postwar era, EMD locomotives dominated mainline freight service with their E and F series locomotives. ALCO-GE in the late 1940s produced switchers and road-switchers that were successful in the short-haul market. EMD launched their GP Series road-switcher locomotives in 1949, which displaced all other locomotives in the freight market including their own F Series locomotives. Eventually GE dissolved its partnership with ALCO and would emerge as EMD's main competitor. By 1960 GE took the top position in the locomotive market from EMD.
Today such diesel locomotives as the EMD SD40, SD70's comprised of the SD70MAC, SD70ACe and the SD70M-2 dominate the railways hauling freight and passengers across the country.