Chennai to Varanasi by train

Hi there,
We would like to travel from Chennai to Varanasi by train, and wonder what our options are here. We are a bit pushed for time but would be open to splitting the journey if it meant we could spend a day or so visiting somewhere worthwhile. Any ideas?
My interests mainly lie in the areas of cultire and religion, although the boyfriend has interests in sport as well, and discovering places a bit different.
Maybe we'd be better just doing the journey in one big overnight go though.

Our rough itinerary is as follows:

Chennai (2 day) - Varanasi (3 day) - Delhi/Agra (3 day) - Jaipur (2 day) - spare for any suggestions (10 days)
Then we fly from Delhi back to Chennai and hope to spend a couple of days there. We have 4 weeks in total.

Any help would be gratefully recieved!