The first letter in all Class Names stands for what gauge the locomotive will run on, officially called “Gauge“. A “W” will denote a locomotive running on Broad Gauge track (1676 mm), Y will tell us the loco will run only on Meter Gauge, Z is 2’6″ Narrow Gauge and N is 2′ Narrow Gauge. Now that all lines in India are being converted to Broad Gauge, non-W locos are becoming increasingly rare. Now, why W, Y and Z instead of, say, B, M and N? During British times, the three gauges for some reason were arbitrarily named X, Y and Z. After independence, X was changed to W, meaning “Wide” for the “Wide” Broad Gauge. The remaining two were left as they were.
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second letter stands for the type of traction used by the locomotive or the power it runs on, officially, “Power“. D denotes a Diesel Locomotive, an A means the loco runs on 25 kV 50 Hz Alternating Current (AC) power, C denotes a loco running on 1500 V Direct Current (DC) power, CA denotes a locomotive that can run on both AC and DC power and B denotes a loco running on battery power. C and CA type locomotives are found only in the Mumbai area as all of the remaining part of the country uses 25 kV AC power. And again, as all DC in the country is being converted to AC, we will not see more of C and CA locos. (Note, CA is considered a single letter)
The third letter specifies which type of service the locomotive is best suited for, called “Load“. P denotes a locomotive specifically meant for hauling trains running passenger services (expresses, mails, local, passenger, anything), G denotes a locomotive best suited for Goods (Freight) trains, if the third letter is M, it denotes that the loco can be used for any service, Passenger or Goods (Mixed), S denotes a low powered shunting locomotive, U designates EMU rakes (local train rakes) which have motors housed in the rake itself and don’t have separate locomotives. There is another one R which denotes a Railcar. The first three alphabets are together called a “Sequence“.
The numbers (4th position) are called “Series“, and denotes different things for Diesels and Electrics. For Diesels the numbering denotes the Horsepower of the engine x 1000 hp. So 2 will denote a locomotive with an engine with more than 2000 hp power rating but less than 3000 hp and so on. This numbering scheme followed today was adopted in 2002 for all Diesel locos other than the WDM2 series. For instance, the WDM2 has 2600 hp power but there is a WDM7 which has only 2000 hp etc. For electric locomotives it is quite straightforward, the series just denotes the chronological version/model of the locomotive. The First AC powered loco was a WAM1 and then came WAM2, WAM3, etc.In this case a new version number can mean a possible new model altogether (WAM3 to WAM4) or a major upgrade from an existing one (WAP1 to WAP4). New version numbers can be also given even to new prototype testing locos which have only small numbers built like the WCM6 (only two in existence), WAM3 (only 10 produced) or WDP3 (Only Prototypes). This is why there are over 50 classes of locomotives in India but only around 15 are widely produced and in service.
The last letter is called a “Subtype” and is the most confusing of all, as it can denote many things from power rating to any unique identifying factor of the loco. For WDM3X diesels the subclass annotation works like this: A denotes 100 hp, B denotes 200 hp, C is 300 hp and so on, F denotes 600 hp. So WDM3F has an engine with 3600 hp power and WDG3A has a 3100 hp powered engine. The WDP4 has 4000 hp power but the WDP4B has 4500 hp and not 4200 as it is supposed to denote and WDP4D is assigned to WDP4B locomotives with two cabs. Electric locos with sub type annotations like WAG5HB (WAG5 with Hitachi traction motors built by BHEL) and WAG9H denote minor modifications to their parent loco classes, like a rebuild, a bit more power, re-geared ratios or some additional components or some other unique identifying factors. The sixth alphabet is sometimes called a “suffix”.