Because of less stoppages and more running time then other express trains WAP 4 is enough upto VSKP. Loco are alloted based on the availability cycle at that point of time. It has nothing to do with premium or prestige. Even with WAP4 it is 45 mins ahead of schedule. I travelled in 1991 then it did BZA MAs in 5hr 45 mins with WAM 16 coaches.