Lonely Planet
Taking the Train from Delhi to Mumbai (What You Need to Know)
Taking the train from Delhi to Mumbai is much more fun than flying. Here
Luxurious train offering tours between Delhi and Mumbai

“The Maharajas' Express is a luxury tourist train owned and operated by The Indian Railway Catering and Tourism Corporation (IRCTC). It serves four routes across North-West and Central India, mainly centered on Rajasthan between the months of October and April.”
Lonely Planet
Taking the train from Delhi to Mumbai is much more fun than flying. Here
Luxurious train offering tours between Delhi and Mumbai