 They needed a higher gear ratio for faster running, the graduated-release feature on the air brakes, and type F tightlock couplers to keep equipment together in the event of a derailment.To fit a steam generator to the freight-only SD40, the designers had to move the machinery forward about on the frame, add a compartment behind the radiators for the boiler, and divide the fuel tank into fuel and water sections.Earlier passenger diesels, like EMD E8, ALCO PA, FM Erie-built and Baldwin Sharknose locomotives, were streamlined cab units designed for visual appeal and the appearance of speed.