The electrification of railways in India started way back in 1925; the year 1966 does not seem to have been a special milestone, so I’m guessing this stamp is one of a set marking “general industrial progress”, a common and international theme of the 1960s.
India has the fourth largest rail network in the world and – from memory – is the largest employer in the world (that might have been a Trivial Pursuit question in the late ’80s). In normal times, some 20,000 passenger trains run every day in India, with over 7000 stations serving cities, suburbs, towns and villages.
https://en.wikipedia.org/wiki/Indian_Railways
Stamp of the Day 