I understand it as far as the diesel engine driving the generator- or is it an alternator?- to power the motors which turn the wheels. But does each wheel have its own motor, or is it one motor per axle?
One electric motor per axle, or one motor per truck. (4 wheels) Diesel turns Alternator AC to Control Panel feeds to DC to motors A significant breakthrough occurred in 1914, when Hermann Lemp, a GE electrical engineer, developed and patented a reliable direct current electrical control system (subsequent improvements were also patented by Lemp).[8] Lemp's design used a single lever to control both engine and generator in a coordinated fashion, and was the prototype for all diesel-electric locomotive control systems.
Earlier diesel-electric locomotives used a d.c. generator and d.c. traction motors. In more recent years - since around the 1990s - a.c. alternators have been used, as suitable a.c. traction motors have been developed. Generally, modern diesel-electrics (and electric locomotives) have one traction motor per axle. I am not completely 'au fait' with US practice, but in Europe and many other countries diesel and electric locomotives are classified by wheel arrangement. Thus, a locomotive with two four-wheeled bogies (or trucks), with motors on each axle is classified as a 'Bo-Bo': i.e., 'B' signifying two axles, with the 'o' signifying that each is motored. A typical US twelve-wheeler is a 'Co-Co', and so on. Earlier locomotives - in the UK, for example - were heavier than those produced today for the same power output. They therefore had additional unpowered carrying axles, denoted by figures. For example, the early main line locomotives had a 1Co-Co1 wheel arrangement: two eight wheel trucks, the leading axle of each being unpowered, with motors on the other three. The mid range locomotives produced by Brush Traction were of the A1A-A1A arrangement, consisting of two six wheel/three axle trucks, each with motors on the end axles, and the centre axle un-powered. We even had some odd-ball five axle locomotives with a 'Co-Bo' wheel arrangement. Mechanical transmission is also used on light shunting locomotives and railcars, whilst hydraulic transmission was popular for a while in Germany and the Western Region of British Railways, but is now used for most diesel multiple-unit trains.
Diesel-electric locomotives most commonly use one traction motor per axle, with the motor shaft parallel to and beside the wheel axle and with a simple pair of gears connecting the two. Early diesel-electric locomotives used a DC generator that directly fed DC traction motors, with the generator often doubling as a starter motor. By the 1970s, locomotives commonly used alternators (which were easier to maintain) that generated AC current, which was rectified back to DC to power the same DC traction motors as before. The EMD GP38 was notably produced with both a generator and an alternator, with the alternator-powered version called the GP38AC. AC traction motors became commonplace in the early to mid 1990s - but they do not use the electricity produced by the alternator directly as it is not of the correct frequency. The AC power from the alternator is first rectified to DC, then (using inverters) back to AC current of the correct frequency for the traction motors. GE locomotives use one inverter per traction motor, while all but the most recent EMD locomotives used one inverter per truck. The traction motors are readily visible on six-axle units such as recent EMD SD series locomotives or GE Dash-9 and AC units. These locomotives all have traction motors that face inwards, so that they all face the same direction on each truck. On the innermost axle they extend considerably beyond rim of the wheels toward the fuel tank, visible in this photo I took of an EMD SD40-2: trainiax /photos/2013/2013-05- To account for the different current and resistance characteristics of the electric motors at low and high speed, locomotives have one or several transitions between wiring the motors in series or in parallel, or in a combination of both.
