Diesel locomotive
Diesel locomotives offer significant operating advantages over steam locomotives. They can safely be operated by one person, making them ideal for switching/shunting duties in yards (although for safety reasons many main-line diesel locomotives continue to have 2-man crews: an engineer and a conductor/switchman) and the operating environment is much more attractive, being quieter, fully weatherproof and without the dirt and heat that is an inevitable part of operating a steam locomotive. Diesel locomotives can be worked in multiple with a single crew controlling multiple locomotives in a single train — something not practical with steam locomotives. This brought greater efficiencies to the operator, as individual locomotives could be relatively low-powered for use as a single unit on light duties but marshaled together to provide the power needed on a heavy train. With steam traction a single very powerful and expensive locomotive was required for the heaviest trains or the operator resorted to double heading with multiple locomotives and crews, a method which was also expensive and brought with it its own operating difficulties.
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.