I used to talk frequently to shop and stall owners in Gurgaon (Sector 17), a simple cold coffee stall made around 2-2.5L/month from a single stall, more than what people with established shops paying 40-50k rent per month were making, stalls made well to do resturants shut down.For chai business you need a good location near offices and customers will be already subscribed to some chaiwala. In the above article that guy makes 50k profit and had to hire people to deliver.
Biryani also sells well but requires some capital and not easy to master.
Momos and similar Indian chinese street food can be started with pretty low capital, very high demand so you do not need a shop at a prominent place from the beginner. Not sure but I think you can even prepare and store them in fridge so less chance of loss.
It was not difficult for anyone with a slightly different street food idea to make a living, most stall owners were earning more than what most graduates earned initially from their job.
PG area is also pretty good for stalls