SD 40

Besides horsepower, the primary characteristic of a passenger locomotive at the time was the ability to provide steam to the passenger cars, for heating, cooking, and sometimes cooling. Secondary characteristics could include a higher gear ratio for faster running, addition of the graduated-release feature to the air brakes, type F tight-lock couplers to keep equipment together in the event of a derailment, or extra signals and communication equipment. Of these, the steam generator was the largest, and had the greatest impact on the locomotive's appearance. To fit a steam generator to the freight-only SD40, the designers had to move all the machinery forward about two feet on the frame, add a compartment behind the radiators for the boiler, and divide the fuel tank into fuel and water sections.