I have slightly different view on this. There should be only 2 types of trains in terms of speed differentiation. 1. SF, 2. Fast passenger. Further difference should be purely based on additional facilities on board and preferred time slot. Take the example of Airline Industry. One type of Aeroplane will cover a distance in same time, whether its operated by full service airline or low cost airline. Such a concept will lead to better utilisation of the Infrastructure and increase in demand.