Indian Railways, in keeping pace with the advanced railways of the world, has been modernizing its railway system for quite some time. The maximum permissible speed on the BG sections of Indian Railways was increased to 120 km/h by first introducing it on the Delhi-Howrah route in 1969. This increase in speed was possible after carrying out extensive investigations and trials on the Rajdhani route using a WDM-4 locomotive and all-coiled coaches. The study was based on the fundamental concept that safety and comfort at high speeds depends upon the interaction of the track and the vehicle. If the suspension system of the rolling stock is very good, then even though track maintenance may be of a comparatively average quality, a reasonable level of comfort will still be achieved. To a limited extent, track stability can be economically introduced on Indian Railways without carrying out major changes in the track structure, by simply selecting better locomotives and rolling stocks and adhering to a slightly higher standard of track maintenance. It was for this reason that the Rajdhani Express was hauled by a WDM-4 locomotive and included all-coiled coaches. The speed of the train, which was originally 120 km/h, has been increased to 140 km/h and Indian Railways is now planning to increase the speed to 160 km/h.
Similarly, in the case of meter gauge, the long existent limit of 75 km/h has finally been overcome and trains consisting of a YDM-4 locomotive and all-coiled Integral Coach Factory (ICF) coaches are running at speeds of up to 100 km/h since December 1997.