Less travel time is the main reason for people to take premium trains and if that travel time is almost same for both premium and non premium trains why will a person opt for premium train ? And also all these trains which we are speaking about are long distance trains in which people book tickets in advanced so if TN gets speeded up and the travel time difference between raj/duronto and TN is just 1 or 2 hrs then obviously most of the people who currently travel by raj/duronto for less travel time will opt for them if and only if TN gets full and there not even chance to get WL ticket also to be confirmed in any class. So that might cause loss to premium trains.