Classification of Locomotives
Q. What do the designations such as ‘WDM-2’, ‘WAP-7’ mean?
Locos, except for older steam ones, have classification codes that identify them. This code is of the form [gauge][power][load][series][subtype][suffix].
In this, the first item, ‘[gauge]’, is a single letter identifying the gauge the loco runs on:
- W = Broad Gauge
- Y = Meter Gauge
- Z = Narrow Gauge (2’ 6”)
- N = Narrow Gauge (2’)
The second item, ‘[power]’, is one or two letters identifying the power source:
- D = Diesel
- C = DC Electric Traction
- A = AC Electric Traction
- CA = Dual-power AC/DC Traction
- DA = Dual-power Diesel and AC Electric Traction
- B = Battery Electric
The third item, ‘[load]’, is a single letter identifying the kind of load the loco is normally used for:
- M = Mixed Traffic
- P = Passenger
- G = Goods
- S = Shunting
- L = Light Duty (no longer in use)
- U = Multiple Unit (EMU/DEMU)
- R = Railcar
The last item, ‘[suffix]’, is an optional indication that indicates something special about the loco, such as a different gearing ratio or brake system than usual.
Steam
The last few models of steam locomotives used in India had this system of classification too, with one change, which was that the ‘power’ code was dropped. Hence: ‘WG’ = BG Goods steam loco, ‘WP’ = BG passenger steam loco, ‘YP’ = MG passenger steam loco, etc. However, there are literally hundreds of types of steam locomotives that have been used in India, and locos classified ‘WG’, ‘WP’, etc. are the exception rather than the rule. Steam locos were classified in a myriad of ways in India, with different systems used by different railways. Some standardization began with the IRS classifications (see below). Note: Sometimes these steam locos had additional notations, e.g., WGx referred to WG locos fitted with CBC couplers for working block freight rakes.
Q. What is the history of the classification schemes for locos?
Early locomotives in India had a bewildering variety of classification schemes. Regional railways had their own classification schemes too. For more details on this, refer to reference works classic 4-volume work on Indian locomotives.
The first BESA standard classes appeared in 1903. The HPS, SPS, HGS, and SGS steam loco classes were quite popular. HP = Heavy Passenger, SP = Standard Passenger, HG = Heavy Goods, SG = Standard Goods. In these, the suffix ‘S’ stands for ‘superheated’. An alternative suffix ‘C’ indicates a conversion to superheating, e.g. SGC. A suffix ‘M’ was sometimes used to mean ‘modified’, for variant designs. However, these classification codes were by no means universally adopted, and various railways had their own schemes.
In 1924, when it was decided to classify engines, the initial notation was:
- X for broad-gauge
- Y for meter-gauge
- Z for 2’ 6” narrow-gauge
- Q for 2’ 0” narrow-gauge
The IRS (Indian Railway Standard) classes XA, XB, XC, XD, XE, and others in the ‘X’ series for BG; YA, YB, YC, YD, and YE for MG; and ZA, ZB, ZC, ZD, ZE, ZF for 2’6” NG; and QA, QB, QC for 2’ NG, were all adopted as standards by the Locomotive Standards Committee by 1925 or soon thereafter.
In fact the Q classes were never built, and of the Z classes, only ZB and ZE (and a modified version of ZF to agree with existing locos) classes were built. Not all locos of a given class were built by the same manufacturer. Some of these class designations were re-used later (e.g., ZD). In 1945, ‘IRS’ became ‘IGR’ (Indian Government Railway Standard), although the class notations remained the same.
‘W’ was used for broad-gauge instead of ‘X’ soon after World War II, with the introduction of the WP and WG locomotives. ‘Q’ was also replaced by the ‘N’ code. Some early electrics had codes beginning with ‘E’ (EF, EM, EG, etc.), but after about 1945, when diesel and electric locos were included in the scheme, the codes for motive power were added (D, A, C, CA, B), which have remained unchanged.
