From today, Google has started offering free, high-speed Wi-Fi service in India's Mumbai Central Station in Mumbai. This kicks off the Internet giant's ambitious plan of setting up Wi-Fi at 400 train stations across the country, in order to expand its presence in India's market.

India has the world's second-largest number of Internet users after China, but only a third of the country's population has Internet access. It is these Indians that Google, and Facebook's controversial Free Basics platform, are planning to connect.

Google CEO Sundar Pichai had first announced that the company would set up Wi-Fi in Indian railway stations after Prime Minister Narendra Modi's visit to its California campus last September. It plans to get 100 of the country's busiest train stations online in 2016, and 300 in its second phase. After Mumbai, Google will start Wi-Fi in the cities of Allahabad, Jaipur, Patna and Ranchi next.

Google has described the initiative as the "largest public Wi-Fi project in the world", and estimates that it will be able to reach 10 million people just by getting 100 train stations online in the first phase. Indeed, while there are free public Wi-Fi projects in the train stations of cities such as Delhi, Kochi, Pune and Bengaluru, their scale remains small and Internet speeds slow.

Passengers at Mumbai Central station will be able to access Wi-Fi on their phone, provided it has an Indian mobile number, and two more devices. The Wi-Fi will be full-speed initially, enabling users to even stream HD videos, but will slow down after the first hour to enable more people to access the Internet.

The service is being offered in partnership with Indian Railways and its telecom wing Railtel. While it will be free to begin with, it isn't clear whether it will remain so in the long run. Google has said that "the long-term goal will be making this self-sustainable to allow for expansion to more stations and places". It remains to be seen whether Google will set a time limit on free Internet usage or explore other revenue models.