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...
more... 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.