Mainline freight service was the last stronghold for steam power in America. By the late 1930s, diesels had proven themselves as switchers and as power for the new streamlined trains, but there were doubts that they could ever be practical in freight service. That all changed in late 1939 when EMD rolled out its brand-new FT locomotive. Borrowing some basic design elements from E-series passenger power, but in a shorter, more powerful package, the demonstrator set out on a 35-state tour that covered nearly 90,000 miles. Railroads were amazed at what this single A-B-B-A lash-up with its incredible 5400 horsepower could do, and orders came quickly. With its proven track record, EMD was the only builder allowed to construct road diesel power during the materials shortages of World War II. Starting in 1945, the basic design underwent a number of changes, resulting in the F2, F3 and F5. But with the arrival of the F7 in 1949, EMD had exactly what the railroads needed to begin dieselizing with a vengeance. The F7 retained the 1500 horsepower 567B prime mover of the F3, but with better electrical equipment, it had a tonnage rating some 30% greater than the earlier model. The units could be geared for freight or passenger service and were equally at home in either assignment. Over the next four years, EMD built over 2200 A and 1400 B units, which were delivered in a rainbow of schemes to nearly every railroad in America. Tough and durable, many soldiered on into the early 70s and several have been preserved.