Don't think...it's such a bad proposition,the Shatabdi between Patna-Lucknow.I guess a detailed analysis of travel/reservation pattern,load factor of trains can throw some light into this.Since distance between Lucknow and Patna is close to 570 kilometers and the distance between Lucknow and Kanpur is less than 85 kilometres.They can very well make that a Kanpur Central - Patna Shatabdi with a total distance of 570+85(roughly) = 655 kms.I have never traveled on that route but i guess it's a very densely traveled route and if there are numbers to back a train like Shatabdi then why not.