Before boarding the train from Nagpur, don't forget to pack your dinner from Nagpur itself. After the train leaves Nagpur, getting food/dinner will be very difficult. So, again warning you, pack your dinner and water, and get some packaged food items like biscuits-chips-bread-fruits before the train leaves Nagpur.
Next day you will get a good amount of breakfast options at Jhansi, Gwalior, and Agra. You can try the hot poori-sabji and chai available at all these stations. I will suggest you to kindly wake up early so that at Jhansi itself you will get sufficient breakfast.
Lunch...
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for the day is not an issue at all as New Delhi will provide a million options for the same. You can even book your lunch online by e-catering facilities or even can order online via dominos, pizza hut, etc like food items.
At Ambala don't forget to try the poori-sabji and other snacks items available on the platform. They are good in taste.
Dinner you can take after alighting at Amritsar.