Dieselisation
Dieselisation took place largely because of the tremendous reduction in operating costs it allowed. Steam locomotives require large pools of labour to clean, load, maintain and run. They also require extensive service, coaling and watering facilities. This was their biggest inferior measure as compared to the diesel locomotive in the number of ton-miles or passenger traffic miles either could run. Diesels could and did have a significantly higher initial price per unit-horsepower delivered. However, their far greater range between fueling stops, the absence of water stops, and the much higher unit availability between inspection repair and maintenance stops, were far better than steam. Diesels simply required significantly less time and labour to operate and maintain. Diesel power was also more scalable to power requirements, owing to the control systems that allowed multiple units to be controlled by one operator. "Double header" steam power required a crew for each locomotive. Initially, diesel locomotives were less powerful than the typical steam locomotives. Between the late 1930s and the late 1950s the power available with diesel locomotive engines roughly doubled, although the most powerful steam locomotives ever built still exceeded the power of the most powerful diesel locomotives from the late Twentieth Century.
Steam diesel hybrid locomotive
In 1925, Jakob Buchli of Switzerland obtained US patent 1559548 for a combined steam and internal-combustion engined locomotive. This differed from the Kitson-Still system in that there was no waste heat recovery and the steam and internal combustion engines had separate cylinders (vertically mounted in the tender), but both driving the same traction wheels. Buchli specified that "…the steam generator is supported upon one vehicle…and the steam and internal combustion engine cylinders together with their driving gear are carried by a separate truck or vehicle". His proposal was for the "steam generator vehicle" to be in the form of a traditional steam locomotive boiler with driver's cab, but without pistons. A "flexible pipe" would lead steam to the pistons within the "detachably coupled…truck" (tender). His claimed advantages were the reduced complexity of a combined transmission system, the improved comfort of the operators being separated from the driving cylinders and the differing maintenance requirements of steam and diesel (such as boiler washouts) being more easily accommodated when the units were detachable. It is not known whether any locomotives to Buchli's design were actually built.
In 1954 Chicago inventor Charles Denker patented a system whereby the exhaust from a conventional four-stroke diesel engine was directed into a large-diameter steam cylinder. There was no boiler: instead a pump, operated by a cam driven from the common crankshaft, injected water (warmed by the diesel cylinder water jacket) into the steam cylinder so that it was instantly evaporated by the hot exhaust gases, powering the piston by expansion. Again, no operational examples are known.