Dieselisation
Weighing against the cost of, and inertia against, replacing the large investment that railroads had in existing steam power were the dramatic increases in flexibility and efficiency with diesel. The fastest and most powerful steam locomotives were faster and more powerful than diesels; however, their range of efficient operation was severely limited. Diesels pro-rate their fuel usage to the length of trains, which a steam engine cannot do. Multiple-unit diesel power is scalable to power requirements with one locomotive crew; steam power is not. A high speed Hudson steam locomotive is good for only one situation, high speeds on level grades. The diesel locomotive can be operated by a single person, with no need of a fireman to run the boiler (two person cab crews may be required for other reasons). Also, diesels use much less fuel and no manpower when idling, something locomotives often do. Diesels can be parked running for days unattended, whereas steam engines must be constantly tended to if not completely shut down. Bringing a steam engine boiler up to operating temperature is often regarded as both an art and science, requiring much training and experience. A diesel is much simpler to start and shut down.
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.