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.
0-6-0+0-6-0
The only steam locomotive example of this type of engine in the United Kingdom was the Leader. It was originally commissioned by the Southern Railway but it was completed by British Railways in 1949. The locomotive was a class of experimental articulated steam locomotive, produced in the United Kingdom to the design of the innovative engineer Oliver Bulleid. The Leader was effectively a Meyer locomotive since both sets of drivers were articulated. It was built in an attempt to extend the life of steam traction by eliminating many of the operational drawbacks associated with existing steam locomotives.
The 0-6-0+0-6-0 configuration was also applied to diesel-electric locomotives when British Rail created the Class 13 in 1965. This was done by permanently coupling two Class 08 0-6-0DE shunting engines as "master and slave" units, the latter with its cab removed. In North American terminology, this is referred to as a "cow and calf" arrangement. The modification came about because of a need to provide more powerful shunting locomotives for the Tinsley Marshalling Yard.