Locomotives: General Information - I

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Classification of Locomotives

Q. What do the designations such as ‘WDM-2’, ‘WAP-4’ 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 (see below)

The fourth item, ‘[series]’, is a digit identifying the model of the loco. Until recently, this series number was simply assigned chronologically as new models of locos were introduced.

Revised class notation for diesels, 2002

Starting in 2002, for diesel passenger, goods, and mixed locos, i.e., WDP, WDG, and WDM sequences, (and only for them, not for electrics, nor for diesel shunters), the series digit identifies the horsepower range of the loco, with ‘3’ for locos with over 3000hp but less than 4000hp, ‘5’ for locos over 5000hp but less than 6000hp, etc. This new scheme was applied to all passenger/goods/mixed-haul diesel locos starting in June 2002, except for the WDM-2 and WDP-1 classes of locos.

The fifth item, ‘[subtype]’, is an optional letter or number (or two of them) that indicates some smaller variation in the basic model or series, perhaps different motors, or a different manufacturer. With the new scheme for classifying diesel locos (see above), the fifth item is a letter that further refines the horsepower indication in 100hp increments: ‘A’ for 100hp, ‘B’ for 200hp, ‘C’ for 300hp, etc. So in this scheme, a WDM-3A refers to a 3100hp loco, while a WDM-3F would be a 3600hp loco.

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.

So, a WCM-2 is a broad-gauge (W) DC electric (C) mixed traffic (M) engine, model 2. Likewise, a WDS-5 is a broad-gauge diesel shunting engine, model 5, and a ZDM-5 is a narrow-gauge diesel mixed-traffic model 5 loco. YAU-1 is the old series of MG EMUs run on the Madras-Tambaram line.

The subtype indication of minor variations is not very systematic. Often successive variants of a model are given subtypes ‘A’, ‘B’, etc. in alphabetic order, e.g. ZDM-5A, WAM-4A, WAM-4B, etc., but not always. In the past, for many loco classes (WDM-2A, WDP-2A, notably), the ‘A’ also indicated dual braking systems (capable of hauling air-braked and vacuum-braked stock). But in some, such as the WDM-2CA, the ‘A’ indicated a loco with only air-brakes. WAM-4P was a version of the WAM-4 designed specifically for passenger use (‘P’). But a WAM-4 6P was a version regeared and allowing for all-parallel operation of the traction motors.

Similarly, a WAG-5HA is a WAG-5 with Hitachi motors (‘H’) built by CLW; a WAG-5HB is the same, but built by BHEL. A WAG-5P, interestingly, is a WAG-5 loco (a goods loco in its original design, as indicated by the ‘G’) which has been modified by re-gearing to haul passenger trains (the ‘P’ indicates ‘passenger’)! An ‘E’ suffix often indicates a variant that is purely air-braked (WAP-1E, WAM-4E, etc., but is redundant these days with all stock being air-braked). A WAG-9H is a WAG-9 with heavier adhesion characteristics, while WAG-9HH is the ‘High Horsepower’ version (9000hp).

Ad hoc combinations of many such suffixes were possible, as with ‘WAM-4 P HS DB 6P’ (HS = high speed, DB = dual-brake compatible, P = regeared for passenger operations, 6P = all 6 traction motors may be placed in parallel operation). The WAM-4 locos, in particular, were notorious for having countless minor variations as CLW and various workshops kept making minor experimental changes to them. Some sheds followed their own schemes too. Bhusawal shed, for instance, added ‘DBC’ or ‘ABC’ to loco classes to indicate locos that had undergone conversion of the braking systems (‘dual brake converted’, and ‘air brake converted’). These days however, suffixes of this type are rarely seen. IR has standardised most of its locomotive classes and their hauling characteristics, so sheds don’t experiment as often.

The model numbers are assigned chronologically as new loco types are brought into use in IR, but there are some exceptions. Sometimes model numbers are assigned to some experimental locos which are never brought into regular use, e.g., WAG-8.

Some sheds have been known to use non-standard classifications; marking a WDM-2 loco as a ‘WDS-2’ to indicate it is used only as a shunter is perhaps one of the more egregiously confusing practices seen in some sheds. ‘WDM-2S’ is another notation seen at some sheds for WDM-2 locos relegated to shunting duties.

In 2009, Golden Rock Workshops had embarked on a program to convert old MG engines (YDM-4’s) to broad-gauge; these were designated ‘WCDS-6’ where the ‘WC’ presumably stood for ‘Broad-gauge, Converted’.

Some classification codes break the system above: e.g., ‘RD’ is used as a code indicating the power and the load for diesel railcars, and not ‘DR’ as one might expect: YRD-1 was a series of MG railcars, NRD-1 similarly an NG series of railcars. Railcars used on the Tambaram line were classified simply ‘RU’. ‘RB’ is used for railbuses, e.g., the WRB railbuses built by BEML for lines such as Bangarpet - Kolar and Bobilli - Salur.

EMUs followed this system for some time through the 1950s, but for quite some time now have not done so. There are numerous different models of EMUs with minor and major variations in use in the Mumbai system and in other systems, and they are no longer distinguished by ‘WCU-xxx’ class codes for them. See the DMU/EMU section for more information on types of these multiple units.

There are many ways in which the classification code appears in IR documents, painted on locos, etc.:

WDM – 2
WDM / 2

The Hindi version is usually a phonetic transcription of the way the classification code would be pronounced in English (‘double-you-dee-em’) with the series number in Hindi.

The serial number of a particular loco usually follows the classification code on the sides and the front or rear (between the buffers) of the loco. See the item on numbering.


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 such as Hugh Hughes’ 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.

Post-independence history of ‘mixed’ vs. dedicated loco models

In the early days locos were classified strictly according to the load: goods engines (G) (e.g. WG, YCG, WCG, WAG, etc), or passenger engines (P) (e.g. WP, WCP, etc). Then the trend was towards a whole fleet of mixed traffic (M) engines (e.g. WDM, YAM, WAM, WCM, etc.) Between 1960 and 1985 or so, almost every loco design was of the ‘M’ variety, with the only two exceptions being the WCG/2 (built 1971 or so) and the WAG series (WAG - 5/5B/5HA/6A/B/C/7). The introduction of the WAP engines in the early ’80s indicated a reversal of IR policy in dedicating engines exclusively for passenger operations once again. Some dedicated diesels (WDP, WDG series) were also then introduced.

Locomotive Manufacturers

Q. Where were/are locomotives used in India manufactured?

Early locos (late 19th century) were almost all imported. The first steam locomotive was built in India in 1895 at the Ajmer workshops.

Details of some of the more important manufacturers are to be found in the section on production units and workshops.

Domestic Manufacturers

CLW: Large-scale loco production in India did not begin until the establishment of the Chittaranjan Locomotive Works (CLW) in 1950. Starting off with steam locomotives, the facilities now manufacture electric locos. More details on CLW here.

BLW (was DLW): The Benaras Locomotive Works (formerly Diesel Loco Works) set up at Varanasi began producing diesel locos in 1967 and did so until 2017, when it began producing electric locos as a result of IR’s policy shift towards 100% electrified operations. Diesel loco production is limited to overseas and industrial customers. More details on BLW here.

BHEL: Bharat Heavy Electricals Ltd. (BHEL) supplied some WAG-5 and three NBM-1 units in the ‘80s, and more recently has suplied WCAM-2’s, WCAM-3’s, WAG-7’s, WAG-9’s and NDM-6’s. It also made the one-off WAG-8 class that never went into serial production. BHEL also makes electrical transmission components, traction motors, alternators, and other components for both three-phase and tap-changer locomotives. The locomotives are made at BHEL’s facilities at Bhopal and Jhansi. In 2008, BHEL entered an agreement with IR to supply several dozen high-horsepower locomotives. It explored foreign partnerships with GE, Toshiba, and others for production of 10,000hp locomotives, but these never came to fruition.

DCW/DMW/PLW: Diesel Components Works, Patiala - first renamed the Diesel Loco Modernization Works or DMW and now Patiala Loco Works, for a long time supplied components for its larger sibling BLW. In the early 2000s, it started works on rebuilding and upgrading locos (e.g., older WDM-2’s being converted to WDM-3A locos). Now, it manufactures electric locomotives (WAG-9, WAP-7 etc.) and diesel locos for industrial customers.

ELF, Madhepura: The Electric Locomotive Factory at Madhepura is a joint venture between IR and Alstom SA to produce the WAG-12 class for use on the Eastern DFC.

DLF, Marhowrah: The Diesel Locomotive Factory, Marhowrah is a joint venture with General Electric for the manufacture of 1000 high-power freight diesel locomotives.

TELCO: Tata Electrical and Locomotive Co. (TELCO) supplied some YP and YG units in the 50’s and 60’s.

Suri and Nayar (SAN), located in Bangalore, have supplied some shunters and other locos, and parts such as transmissions.

Ovis Loco, based in Hyderabad supplies some shunting locos (mainly for industrial customers).

Cummins India makes the diesel engines for DMU’s, HPDMU’s, and some MG locos.

NGEF and Crompton Greaves are other domestic suppliers of traction motors, alternators, etc.

Medha Servo Drives are probably the main supplier of many electric locomotive components including traction motors, driver consoles, hotel load convertors. They are also producing the power train for the upcoming T-18 trainsets.

Bombardier Transportation (now Alstom) makes train-sets for many metro systems in India from its plant in Vadodara, Gujarat. It also makes drive trains for Mumbai area EMUs.

ICF: Integral Coach Factory has been making EMUs for suburban systems since 1966. It also makes some self-propelled special-purpose units, such as the diesel Medical Relief Van and diesel-electric tower cars. More details on ICF here

Bharat Earth Movers Limited (BEML): Formerly a state owned enterprise, Bangalore-based BEML manufactures heavy engineering products, including a wide variety of Railway vehicles made at its Bangalore and Mysore facilities. It used to make a lot of passenger coaches based on ICF type design, but recently most of its focus is on trainsets for the Delhi and Bengaluru Metro systems. DMRC rolling stock is manufactured in technical collaboration with the Korean firm ROTEM. BEML also manufactures Railbuses, OHE vans, track-laying equipment and some types of freight wagons.

The Jamalpur Workshop of ER manufactures diesel self-propelled units such as cranes for clearing wreckage and construction work.

The Izzatnagar Works manufactures railbuses and railcars using Ashok Leyland Iveco engines, and Hindustan Motors or Kirloskar Pneumatics converters and transmissions.

Jessop & Co., an engineering company founded in 1788 has manufactured wagons, cranes, and EMU rakes (many of the old EMU rakes in the Calcutta and Mumbai systems were built by Jessop).

Parel Workshops of CR has been assembling BG and NG diesel locomotives since 2006, mostly WDS-6 class shunters, using components from DLW, DCW, and elsewhere. While IR remains a customer, most of its products are for industrial concerns.

Foreign Manufacturers

There are many foreign suppliers of early locos: dozens of manufacturers are represented, including Bagnall, Beyer-Peacock, Vulcan Foundry, Nasmyth Wilson, SLM, Kerr-Stuart, Henschel, Hunslet, Dubbs & Co. (Glasgow), Metropolitan Carriage and Wagon Works, Sentinel Carriage and Wagon Works (Shrewsbury), Freid Krupps, Sharp Stewart, Kitson-Meyer, Kawasaki, General Electric, etc.

More recent foreign suppliers of locos include Alco (WDM-1, some WDM-2, some YDM-4), GM-EMD (WDM-4, YDM-3, YDM-5, WDP-4, WDG-4), English Electric (WCM-1, WCM-2), Henschel (WDM-3), Maschinenbau Kiel (WDS-3, ZDM-2), Montreal Loco Works (YDM-4A), Hitachi (WCM-3, WCM-4, some WAG-2), Mitsubishi (WAM-2, WAM-3, YAM-1, some WAG-2), Asea Brown-Boveri (WAP-5, WAG-6A, some WAG-9), Toshiba (some WAG-2), Krupp (some WAM-1), Alsthom (some WAM-1), Kraus-Maffei (some WAM-1), and Arn. Jung (NDM-1).

Cranes and other equipment have been supplied by Gottwald, Sheldon, ABB, etc. Industrial locomotives of various kinds were supplied by Andrew Barclay, Brookville, Baldwin, Henschel, Canadian Loco. Co., Greenwood Batley, Ruston & Horsnby, TELCO, Arn. Jung, Kraus-Maffei, GE, and many others.

Soviet Locomotives

Post-Independence India had a pretty close relationship with the USSR, and had access to a wide range of Soviet technology in many fields. Interestingly, though, Soviet influence was extremely limited in the area of locomotives, which was dominated by European and American suppliers as mentioned above. The Soviet Union helped build some of the steel plants and other industrial sites in India in the 1950s and 1960s, and a few industrial locomotives did seem to have made their way to India from the USSR. In particular, 36 type TEL Bo-Bo DE locomotives, of 5’6” gauge, variants of the TGM-3 750hp end cab switchers are listed in some locomotive compilations as intended for India. These were built around 1957 at Lugansk. A TEV version appears to have been built at 1958, also at Lugansk. While it is not entirely certain, it seems likely these were intended for the Bhilai Steel Plant which was built with Soviet help and commissioned in 1959. Iron ore for the plant was mined at Dalli-Rajhara and transported over a 85.5km railway line to Bhilai, which opened on May 14, 1958. A 20km line connected Bhilai to Ahiwara where limestone was quarried; this line was opened on April 1, 1960.

(Information from the WDL mailing list, posted by John Middleton. Reference: Vitaly Rakov, Locomotives of our country’s railways, 1995, Moscow, ISBN 5-277-00821-7.)

Early Non-Steam Locomotives

Q. Which were the earliest diesel locomotives in the Indian subcontinent?

In 1915, a 2’6” gauge diesel loco was supplied to the India Office by Avonside (Bristol). This is presumed to have worked on some tea plantation in Assam.

In 1921, a 2’0” gauge 0-4-0 diesel loco built by Baugleys of Burton-on-Trent was delivered to Bengal.

In 1923, two diesel locos built by Ruton Proctor of Lincoln were used on the Barsi Light Railway.

In 1929, a 2’0” gauge 0-4-0 diesel loco was supplied by Maffei (Germany) to C K Andrew, London, for delivery to India. Their ultimate use and disposition in India is not known.

Two 420hp (or 350hp?) dual-cab BG diesel shunters from William Beardmore (with electrical components from GEC) were used by the North-Western Railway (now in Pakistan) in 1930.

In 1934, an Armstrong-Whitworth diesel-electric railcar was delivered to NWR for use on the 2’6” Kalka-Simla Railway.

In 1935, two 1200hp BG diesel-electric locos with 8-cylinder Armstrong-Sulzer engines, built by Armstrong-Whitworth, were obtained by NWR for trials on the Karachi-Lahore line in preparation for a proposed new Karachi-Bombay route. They had a 1A-C-2 wheel arrangement. None of these experiments proved successful and the locos were in all cases withdrawn very soon.

Ceylon Government Railways obtained one diesel-electric shunter and two diesel-electric mixed traffic locos in 1934 from Armstrong Whitworth. The two mixed traffic locos were actually made for the Indian State Railways (as they were then known) but turned out to be of too low a power for their requirements and were sold to CGR. These proved unsuitable and were sent to Argentina in 1937, and ultimately scrapped soon thereafter.

Between 1930 and 1940, various Indian industrial concerns obtained 14 diesel locos. Bagnall in conjunction with Duetz supplied a 4-speed 22-25hp diesel loco (Duetz PM 2117 design) in 1934 to Bundla Beta Tea Co., Assam for the Pengarie-Digboi trolley line. It had a top speed of about 15km/h. In 1936, BBCI obtained one diesel-electric shunter from Armstrong-Whitworth which survived into the 1950’s. In 1940, the Jamnagar and Dwarka Railway obtained one MG diesel from Brookville (USA).

In the mid-1930s, the Nizam’s State Railways obtained a few diesel railcars from Ganz. These were in use until the 1950s or so. Ganz supplied NWR in 1939-40 with some diesel railcars as well. Some of these were allocated to India when NWR assets were split following the partition of British India.

In 1944-45, the USATC supplied 15 GE-built BG Bo-Bo diesel locos with Caterpillar engines to WR. These were mid-cab machines with short and narrow hoods on either side. Several of these were working until about 1990, when they were withdrawn and scrapped. One is preserved at the Benaras Loco Works, Varanasi.

In 1949, a few MG diesel-mechanical locos built by Fowler were imported by IR for use in the arid regions of Saurashtra. One of these is preserved at the National Rail Museum, New Delhi.

In 1955, 20 ‘DY’ diesels by North British were imported for use, also in Saurashtra (MG). The locos of the second batch were reclassified as YDM-1’s and survived well into the early 2000s.

Q. Which were the earliest electric locomotives in India?

Two MG electric locos using overhead electrification were supplied to the Mysore Gold Fields in 1910 by Bagnalls of Stafford. Electrical equipment for these was supplied by Siemens.

Electrically operated rail trolleys (patented by T A White, an EIR engineer, and hence known as White’s Patented Rail Motor Trolleys) were used in a few places beginning in 1910. EIR’s Liluah Carriage and Wagon works used one between Liluah and Howrah; the Oudh and Rohilkand Rly. also used one for track inspections. In the following years Jessop and Co. supplied a few more of these to various railways.

In 1922 an electric loco (unknown gauge) using overhead electrification was supplied to the Naysmyth Patent Press Co. in Calcutta by British Electric Vehicles.

Q. Other than diesels, were there other internal-combustion locos used in India?

For contemporary applications, see the section on alternative fuels.

In 1905 Kerr Stuart delivered a 12hp 0-4-0 petrol-driven 2’6” loco to Morvi Railway and Tramways.

In 1909, a railcar with a Dodge petrol engine was supplied to the Matheran Light Rly.

In 1909, a 0-6-0 petrol-driven MG loco was supplied by McEwewn Pratt and Co. of Wickford in Essex to Assam Oil Co. In 1910, Morvi Railway and Tramways obtained a 30hp 0-4-0 petrol-driven 2’6” loco from Nasmyth Wilson. In 1910 and 1911, a few petrol-driven parcel delivery vehicles were supplied to EIR by Thornycroft. Another 13 petrol-driven locos were delivered by various builders up to 1920, and a further 75 between 1920 and 1930. Their use started declining after that, and only 27 more were ordered later (until 1940).

Some petrol-driven railcars were built by the Motor Rail and Tram Co., Ltd., and supplied to the South Indian Rly. in 1925. Their engines were rated 65bhp at 1000rpm; a railcar seated 85. They were refitted with diesel engines in the late 1930s.

The Shahdara-Saharanpur 2’6” Light Railway had a petrol railcar supplied by D Wickham Co. in 1935.

A Brookville petrol locomotive was used by the Matheran Light Railway in 1928. Two railcars (one seating 8, the other seating 14) with Dodge petrol engines and chain drives were also used by this railway (1909, 1927).

Three alcohol-fuelled MG locos with mechanical transmissions were supplied by Davenport Locomotive Works (Iowa, USA) to some unknown Indian industrial concern in 1949.

Continue on to read about serial numbers and wheel arrangements.