Premier trains like the Shatabdi, Rajdhani and Duronto Expresses include food in the fare of your ticket and serve it to you at your seat. On other trains, food needs to be purchased separately.
There are no restaurant cars on Indian trains - you can buy food from passing vendors. Alternatively, an attendant will usually appear before mealtimes, asking you whether you'd like a meal and listing out the options available, usually limited to "veg" and "non-veg", which is delivered to your seat a little later.
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