In both the diesel as well as the electric engine, the primary intention is to supply electricity to traction motor (which may either be a 3 phase Induction Motor or a DC motor). These traction motors are connected to wheels, which make the train move.
The first stage of every electric engine is same: Pantograph.
Pantograph consists of collector bars, which touch the overhead lines and collect electricity. This then runs over bus bar on the roof of locomotive, through surge arrester and vacuum circuit breaker (These are devices used to protect the locomotives from short circuit/ overload currents/ effects of lightning etc).
The output from circuit breaker is fed to transformer.
From here, depending on whether your locomotive has DC motors (and tap changer operation) or Induction motor (and VVVF controls) the process varies.
Locomotives with Tap changer
In locomotives which has tap changer, there are two transformers, wound on same core. The first one is an auto transformer with 31 tappings. depending on where you tap, the output voltage from auto transformer varies and this varying of taps is done in the locomotive cab. The output of this auto transformer is fed to another transformer, which is a step down transformer with fixed ratio. It steps down the voltage to a safe, operating level for the semiconductor devices to whom the output of transformer is to be fed.
From here the output of transformers is send for rectification (that is conversion from AC to DC) as we have Dc motors. The output of this rectifier blocks is fed to filters where the ripples from rectifier output are smoothened out and fed to DC switchgear and combination control of traction motors (as there are multiple traction motors) and then to the traction motors which are operated by DC.
The control of speed/torque is by means of tap changer which changes voltage (and thereby current) that is fed into the system.
Locomotives with VVVF control
Here the output of transformer is fed to a power converter stage, which concerts AC to DC (convertor is another name used for rectifier). These convertors utilize either GTO’s (old technology) or IGBT (current technology) for this purpose. The output is filtered by another circuit to provide a fairly flat Dc output. We further have a stage to trap harmonics arising in the system and stages to improve power factor of the system.
After this the output is fed to an inverter stage which converts the DC to a three phase Ac, which is then fed to the traction motors.
Here control is by means of VVVF controls, which varies both frequency as well as voltage. Control on Induction motors is quite difficult, but the recent advent of power electronic devices has made frequency control quite easy.
The output of traction motors is not directly coupled to wheels. We have reduction gears placed over there which reduce speed (while increasing torque). The variation in ratio of reduction gears enables us to use same traction motor for both freight and passenger locomotives.
Source: Quora by , electrical engineer.