There are three ways of generating electricity for the coaches :-
1)SELF GENERATION (SG) : Individual coaches are powered by axle-driven generators which charge storage batteries that power lights, fans and other electrical fittings. Older coaches have 24V (less often 48V) circuitry and have dynamos connected to the axles by belts. Newer coaches have 110V circuitry and use belt-driven 4.5kW, 110V alternators. Both systems use banks of 24V batteries (mostly lead-acid batteries of an 800Ah capacity) for back-up power.
Note : Self generation is a very inefficient method of generating electricity.
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SG alternator in a non-AC coach generates about 4.5 kw, ie 6HP , but absorbs over 10 Hp. In Ac coaches it generates 25kw or 32 Hp,but puts a load of over 45 Hp on the axle.
In self generation, you transmit mechanical energy into electrical energy and the process has about 65-70% effeciency.
You store this energy in batteries whose disposal is pollution, and are a re occurring cost, and also in effecient.
The mechanical energy from axles means there is more stress on couplings, and also more power and TE that the loco has to apply on the rails.
2) END-ON GENERATION (EOG) : In Rajdhani/Shatabdi/AC Duronto trains and a few others like the Garib Rath Expresses, the provision of dedicated rakes allows the use of a separate 'power-car' to supply electricity for all the coaches. There are usually 2 generators in each power car; each generator (an End-on Generator (EOG)) generates 3-phase 750V AC power, which is then distributed across the train, and stepped down to 415V AC (3-phase) for the air-conditioning, or 110V (single-phase) for other appliances. The elimination of generation equipment also allows the coach bogies to be designed with higher speeds in mind. The power car capacity is 250kVA (older models) or 500kVA (newer models, 'high-capacity power cars'). For the higher-power EOGs, often each power car at one end of the rake provides power when the train is running in one direction while the other operates in the other direction. The lower power EOGs can usually power up to 18 AC coaches, but their peak efficiency is at a load in the range of 7 to 12 coaches, and so for longer trains both EOG cars are on simultaneously. The two EOGs and the coaches along the length of the train are connected by two independent sets of 3-phase cables so as to be able to handle a failure in a cable. In addition, there are usually 24V batteries in the coaches to power a couple of emergency lights at critical points in the coaches.
Note : In EOG operation, EOG engines remain at high effeciency level between 40% load to 75% load condition, if load is below 40% one generator should be ideally switched off. Here there is only one form of energy conversion, so loss of efficiency at only one place. So cost of energy per kw is comparatively lower.
3) HEAD-ON GENERATION (HOG) : Another system, Head-On Generation (HOG) has been under research by IRIEEN and RDSO but not deployed yet to any trains. In this, power for the hotel load of the train is taken directly from the OHE through a separate pantograph mounted on a power car, or through a special separate hotel load winding tap provided in the main transformer of the locomotive. Locomotives such as the WAP-5 series already have the provision for the hotel load tap. A separate power car is still needed when taking the locomotive tap for hotel load power, because a transformer must still step down the power drawn for distribution to the coaches.
If using a separate power car with a pantograph, the placement of the power car within the rake is likely to be at the rear to ensure safe inter-pantograph distance between it and the pantograph(s) of the locomotive and simultaneously to minimize coach shunting for forming the rake. Mechanisms like Locotrol need to be used to raise and lower the pantograph remotely from the locomotive cab.
Whether the power is drawn from the OHE or from the locomotive tap, it still needs to be further converted to 415V 3-phase / 110V 1-phase as required for the coach air-conditioning and lighting systems. This can be done in a Bulk Coach Converter in the power-car, or in individual coach converters provided in each coach (or in every two or three coaches.
Note : HOG is the most efficient form of energy for the coaches, here the OHE power in electrics and main engine power in diesels is used by the rake, while decelerating the power otherwise wasted in dynamic bakes is utilized by the HEP invertor.