As already described for diesel-mechanical locomotives, internal combustion engines can only be effectively combined with a mechanical gearbox at lower outputs due to their starting behavior and the dependence on the rotational speed. For mainline locomotives and larger railcars, a possibility was sought to completely decouple the engine speed from the axle speed in order to allow the engine to run at the optimum rotational speed at any running speed.
Shortly after the introduction of the first electrically powered trains with overhead lines, the idea of carrying the generator on the locomotive arose. This would make it possible to dispense with the expensive overhead line and prevent transmission losses. The first rail vehicle of this type was apparently “La Fusée Electrique” by Jean-Jacques Heilmann. In this case, the piston steam engine drove a generator that supplied electricity for the eight traction motors. After this design was unsuccessful, many railcars and light locomotives were built from the turn of the century that still had electrical power transmission in connection with a petrol engine. With the 57-ton gas-electric boxcab, General Electric built the prototype for the first line locomotive with electric power transmission in 1913.
Some diesel-electric locos, like this British class 37, have stood the test of time in service for many decades
BR_37_Colas 
After the diesel engine had established itself as a high-torque and economical alternative to the petrol engine, the first diesel-electric mainline locomotive ran in 1924. This was the Ээл2, built by Maschinenfabrik Esslingen according to plans by Yuri Vladimirovich Lomonossow for the Soviet State Railways. In the USA, ALCO began series production of larger diesel-electric switcher locomotives in 1931, while the Deutsche Reichsbahn had diesel-electric express railcars manufactured from 1932.
In the years that followed, the control of diesel-electric drives was further developed. Here the challenge ist that when the power is being switched on, the excitation of the generator and the speed and/or charge of the diesel engine has to be increased. The breakthrough came with the servo field control, which the Swiss company BBC had presented in 1935 and which was subsequently installed in many locomotives and railcars.
Modern diesel locomotives, such as the Stadler EuroLight or the British class 68, have three-phase motors
Train Photos / www.flickr.com/people/99279135@N05 In the USA in particular, diesel-electric locomotives were already selling well in the 1930s. After the Second World War, they were used to replace steam locomotives on a large scale, and several companies outside of North America also secured the license rights to the propulsion technology of the US diesel locomotives. While diesel-hydraulic power transmission was used in Germany in particular, electric power transmission prevailed worldwide in diesel locomotives. The US manufacturers in particular exported their products to large parts of the world. But British companies such as English Electric were also able to deliver many diesel-electric locomotives, especially to the former colonies.
From around the mid-1990s, power converters and three-phase traction motors found their way into diesel locomotives, where computer-aided control ensured optimal operation in most speed and load ranges. The six-axle US diesel locomotives with three-phase motors now develop a starting tractive effort of more than 180.000 pounds with specially adapted bogies. The German manufacturers are now also increasingly building diesel-electric locomotives again, because this makes it easier to implement a common platform with electric and diesel locomotives. It is also possible to build dual-power locomotives with a combined diesel and electric drive, in which the axles are driven by electric traction motors.