In the steam era the maximum speed on Indian Railways network barely exceeded 80-90km/h. It was only with the advent of the WP class of 4-6-2 Pacifics in the late 1940s did trains realistically approach the 100km/h mark. The WP was the mainstay of the IR passenger train haulage around the country. While electric locos were operational in and around the Bombay area right from the early part of the 20th century, they too never really crossed the 100km/h figure.
The first development in traction beyond steam happened when electrification was introduced in Bombay. From the turn of the 20th century, electric locos running on 1.5kV DC current had been operational in an around Bombay. The first of these, a WCP-1 loco with an unusual 2-Bo-1A1 bogie layout had a theoretical top speed of 120km/h. These were introduced on the famous Deccan Queen Express and cut down the time taken to travel from Bombay to Pune from 6 hours to 2:45 hours.
Many other English and Indian built classes were introduced on the Bombay division but the top service speed never exceeded 105km/h. The intervening World Wars stopped the electrification drive from expanding beyond Bombay and steam remained India's primary mode of traction till the 1960s. The electric hauled Deccan Queen and the steam hauled Taj Express between Delhi and Agra were among the rare trains in India to regularly touch and exceed 100km/h during those years.
The era of diesel locos dawned in the 1960s. The ALCO (American Locomotive Company) built WDM-1 class was the first mainline broad gauge diesel loco to operate in India. Derived from the DL500 World Loco series, these machines struggled to cross 100km/h with anything above a light load. It was only when IR imported two new classes of Broad Gauge locos from America, the ALCO built WDM-2 and the GM-EMD built WDM-4 class. Both these locos produced in 2600hp (2400hp at the wheels) and with their arrival in 1962 did IR look to bump up the speeds of its trains.
The first incremental step happened in 1967 when both the WDM-2 and WDM-4 were allowed to haul Mail / Express trains at 105km/h. 35 kg rails and 8" depth ballast was the pre-requisite for this increment. One must remember that most tracks back then were laid on wooden or metal sleepers.
The real push towards high speed train travel came with the introduction of the Rajdhani Express in March 1969. The first train of this class ran between New Delhi and Howrah with an all AC accommodation. The coaches were specially modified with an all coil suspension in the bogies and were powered by generator cars distributed across the length of the consist. This dispensed with the need for heavy batteries making the whole train lighter. On the upgraded Grand Chord (Delhi - Allahabad - Gaya - Howrah) track section this train was permitted to run at 120km/h with a WDM-4 class locomotive doing duty.
The concept of a Superfast Express train was also introduced in the early 70s. In 1971, trains like the Grand Trunk Express between Delhi and Madras, Frontier Mail to Bombay, the Howrah Mail between Bombay and Calcutta were permitted to run at 110km/h with a WDM-2 locomotive. In the same year, the Howrah Rajdhani's speed limit was uprated to 130km/h.
In May 1972 a second Rajdhani train was introduced between New Delhi and Mumbai Central via Mathura - Kota - Baroda. This train was allowed to run at 120km/h for most of its journey and was hauled by a pair of specially modified WDM-2 locos. Their axle boxes were fitted with devices known as "Resilient Thrust Pads" allowing for the increase in speed. Over the years this feature became standard across the class allowing almost all units to run at 120km/h if required.
To enable running at such speeds, the Research Designs and Standards Organization (RDSO) developed what are known as C&M (Vol 1) standards. They specified that for high speed travel, tracks should be laid with a minimum of 52 kg (weight of rail per metre) rails on ballast at least 250mm in depth out of which at least 100 mm had to be clean, raked, compacted and in stable formations.
Throughout the 1970s, electrification projects were also started on some of the other trunk routes in the country. However, the available crop of electric locos was not capable of operating the high speed trains. While trials were conducted in 1971 with WAM-2 and WAM-3 locomotives on the Howrah - Durg and Howrah - Kanpur sections at 140km/h, they were certified only for 110km/h operation. Even the more powerful WAM-4 class of locos were restricted in the 100-110km/h band.
The breakthrough was made in the early 80s when CLW developed the WAP-1 class locomotive. The design mated the WAM-4 class' TAO69 traction motors in 2S-3P formation and a gear ratio of 58:21 housed in a semi-streamlined shell. This locomotive designed to haul an 18 coach Rajdhani Express at 130km/h. The original prototypes were designated WAM-4B and were tested between Ghaziabad and Kanpur for straight line speed at 145km/h and on curved tracks between Gomoh and Gaya at lower speeds. Certified fit for 130km/h operations, the locos were pressed into service in 1982 with the Howrah Rajdhani Express replacing the WDM-4 class.
In 1983, tests were also carried out with twin WAP-1 locomotives to assess any emergency braking distance improvements. This was the period when Indian Railways was switching over from vacuum braked rolling stock to air braked stock. The Howrah Rajdhani was the first train to feature an air braked consist and these tests were designed to assess any improvement with twin locos. The trials were inconclusive as a single loco with 18 coaches produced results similar to tests with twin locos.
Though the train ran with twin locos for a short period around 1983 it reverted to being single loco hauled soon after. A variant of the WAP-1 class for higher speeds was developed. Called the WAP1-FMII, this loco featured fabricated Flexicoil MkII bogies instead of the cast bogies of the regular loco. In 1988 these locos were tested on the Ghaizabad - Tundla and Tughalkabad - Agra sections at 160km/h and were certified for commercial operations at 140km/h. The first Indian Train to reach this speed was the newly introduced Shatabdi Express between New Delhi and Jhansi in 1989.
The WAP-1 FMIIs were renamed as WAP-3 but soon they developed fatal problems. The FM-II bogie was fabricated, i.e. welded together from pieces unlike the single piece cast bogie used in the WAP-1. The FM-II bogies started developing cracks while on the run and pretty soon it was apparent that the design was not successful. The 140km/h permissible speed for the Shatabdi was brought down to 130km/h and by the mid -90s, all WAP-3s had their bogies replaced with standard ones and the locos were renamed WAP-1.
The WAP-1 locos however were a stop-gap arrangement. The failure of the WAP-3 experiment along with their 2S-3P motor pairing and high gearing meant that acceleration at speeds beyond 90km/h was slow. And with growing traffic and train loads a more powerful loco was needed. RDSO went back to the drawing board and came up two designs which were meant for 160km/h operations.
One of these locos was diesel powered while the other was an electric. The first of these to enter service in 1995 was the WAP-4 class developed from the WAP-1 design. It featured Hitachi HS15250 motors rated at 5350hp (5000hp cont.) mounted in Flexicoil Mk1 cast bogies in a streamlined shell. The loco was an instant hit and by 1996, had been cleared for 140km/h. In commercial service the loco was restricted to 130km/h due to track and coach limitations. In trials, the loco had touched 169km/h, a record for Indian Railways at that time. Improved constantly, these locos are a mainstay of the long haul fleet of Indian Railways to this day and are renowned for their reliability.
The other loco was the WDP-2 (later renamed WDP-3A). Featuring an upgraded version of the WDM-2 prime mover, the loco produced 3100hp. Housed in a streamlined twin cab shell featuring fabricated Flexicoil MkIV bogies, the loco was tested and certified for operations at 160km/h. However, the loco never met the design goals in reality and produced massive lateral movement at speed. Only 65 odd were produced and the fastest speed the loco managed in commercial service was 120km/h hauling the Trivandrum Rajdhani between Vadodara and Virar and the Konkan Railway section.
RDSO's obsession with the Fabricated Flexicoil MkIV bogies did not end with the WDP-2. Alongside the WAP-4, it also developed the WAP-6 which was identical in design to the WAP-4 but with the aforementioned bogies and a commercial speed of 160km/h. Needless to say that this experiment also failed and the 15 WAP-6s had their bogies changed to the cast Mk1 bogies and renamed WAP-4 by the early 2000s.
The Indian Railways boffins rightly recognized that the future for DC motored locos was limited. AC motor technology was getting popular the world over and the first such loco to make it way to India was the WAP-5 class. Designed and built by ABB, these Bo-Bo locos weighed only 78 tonnes, but packed quite a punch. 5000 HP is delivered to the wheels through fully suspended three-phase traction motors supplied by GTO based converters which are controlled by advanced micro-processor based control technology. They were imported for service on fast, short trains like the Shatabdi Express. The locos also featured fully suspended traction motors reducing the impact on tracks and allowing faster speeds.
The first batch of 11 locos hit Indian shores in 1995 and was initially certified for 130km/h operations. In June 1997, trials were conducted between Khurja and Somna where the loco hit a top speed of 184km/h. This remains till date, the fastest recorded speed on the Indian Railways. The WAP-5s were put into service on premium trains like the Shatabdi and Rajdhani services. Of the first 11 locos, one was damaged beyond repair in transit. In 2000, the first indigenously built WAP-5 entered service and nearly 70 such locos are in operation today.
The dawn of the 21st century saw the entry of two heavyweights in the arena. The first was a diesel, marking the return of GM-EMD to India. The WDP-4 locomotive or the GT46PAC as its manufacturer called it, was a 4000hp 2 stroke diesel loco with AC traction motors. The loco had 6 axles out of which only the outer four were powered in a unique Bo-1-1-Bo layout. It was the passenger version of the 6 motored GT46MAC loco imported by Indian Railways and named WDG-4 locally. The loco was tested on the Konkan Railway in December 2002 with an 8 coach load and attained a maximum speed of 165km/h and was able to maintain 150km/h in sustained runs. Later in the month, further trials were conducted in Ghaziabad - Tundla section where the WDP-4 attained 180km/h, the fastest speed recorded for a diesel locomotive in India.
The WDP-4 design went through several modifications, wherein a 6 motored 4500hp variant named WDP-4B was successfully developed and production of the 4 motor WDP-4 stopped. Another variant called the WDP-4D with twin cabs mounted on the WDP-4B design was developed and certified and the production of WDP-4Bs has also stopped. However, with diesel hauled trains on high speed routes being a handful, the WDP-4 class does not see commercial speed in excess of 110km/h on most sectors.
The year 2001 saw another significant development. German designed and built Alstom LHB coaches were introduced on the Swarna Shatabdi Express between New Delhi and Lucknow. Featuring a lighter shell with greater capacity, modern two stage suspension, rigid CBC couplings and disc brakes, the coaches were certified fit for 160km/h operations after extensive trials. However, they went into commercial service restricted at 130km/h owing to track and traffic restrictions. The LHB design was soon extended to sleeper shells as well and in 2003, the Mumbai Rajdhani started running with LHB rakes at 130km/h.
Along with the WAP-5, Indian Railways had also imported the WAG-9 class 6000hp Co-Co freight locomotive. This design was adapted for passenger operations indigenously resulting in the birth of the WAP-7 class locomotive in 2000. Developing 6350hp via 6FRA 6068 traction motors, this 123 tonne loco was capable of accelerating a 1430 tonne load to 100km/h in 235 seconds as compared to 325 seconds for a WAP-5. The locomotive was certified for mainline operations in 2003 at 130km/h and took charge of hauling some Rajdhani services from the WAP-5.
In January 2006 trials were conducted with LHB coaches on the Tughalakabad - Agra section with a WAP-5 hauling LHB coaches. The trials were successful and in the same year in October the Bhopal Shatabdi (the new avatar of the Jhansi Shatabdi) was allowed to run at 150km/h between Tughalakabad and Billochpura stations of the New Delhi - Agra section. To this date, this remains the fastest speed a commercial service is allowed to achieve on Indian Railways.
Efforts are on to allow other trains to reach these speeds with LHB coaches. At present, as an experiment the Mumbai Rajdhani and the August Kranti Rajdhani Expresses are permitted to run at 140km/h between Ballabgarh and Palwal stations. If certified these trains will run at 140km/h on the Tughlakabad - Kota sector in the near future.
Indian Railways is also experimenting with running twin WAP-5 locos hauling 24 coach trains at speeds of up to 160km/h. Owing to catenary design limitations, it modified the locos in 2013 to draw power from a single pantograph with power to the secondary loco provided via a high tension cable. These locos have not yet entered commercial service.
With Indian Railways gradually moving its entire production to LHB coaches and three phase locos, it is expected that the 110km/h limit for most trains will be revised in the near future and regular trains will operate at 120-130km/h while premium services will run at 140-160km/h. There is also talk of importing and introducing EMU style trainsets on Rajdhani services to replace loco hauled consists. With these steps, it is expected that average speeds of Indian trains will rise significantly from the current 45-70km/h band to the 60-100km/h band.