Please define "huge". Googling shows me that Google's wifi was setup at mass transit stations. Wikipedia says: " In 2014-15, IR carried 8.397 billion passengers annually or more than 23 million passengers a day "
23 million people a day is not a trivial number.
If people who use IR use it on average once every ten days (most may use it daily?), you'd project 230 million people would use IR in a year.
That's still only a fifth of the Indian population; I would call 80% a "huge" percentage.
Really, though, the point is that nobody spends much time at railway stations (at least, not a majority of their day), so even if everybody used the wifi once, it wouldn't have a major effect on their lives.
Compare that to 2G/3G, which blankets much larger swathes of land – places people actually live and work.
Telcos begrudge the cost of servicing the rural areas, and are concentrating a majority of their data dev to help metros.
The Facebook freebasic fiasco was also targetted at metros and young teens. They know which target markets would respond to free basic.
The high sounding promises were rhetoric - studies on who used the data showed that the market was users who used it to connect to other facebook users for free - college students.
Most of India is not educated in English, or literate - full internet which allows the sharing of images (not possible on FB) is significantly more useful.
In India, IRCTC (railways website) is used a lot. so having free connectivity on a train station is actually very useful. It may not be blanket coverage but it is practical, targetted and impactful coverage.
India needs more of that kind of work, because investment is slow and money hard to come by. Solving multiple use cases at one go with low resources is a great solution.