my friend it does not depends upon engine that much it depend how the route is as you can see bhopal to delhi and bomay to delhi is the luxorious route and there are some speed restriction on every route.

---

I guess all the trains use almost same kind of engines. The speed of any train is determined by the status of train (superfast or passenger). The duronots dont have any special engines. They get the clear signals and permission to go that fast. thats it. i hope so.

---

my friend...all superfast train engines can only go upto a max, speed of 110 kph...while for most of rajdhanis this speed is 130 kph.....

---

yes you are correct to some extend that the first preference is given to duronto,rajdhani, shatabdi and after thar mail/exp and then passengers. but you can't run any train at speed at any route there are some speed restriction. the routes are divided into several categories such as bombay-delhi A class route and howrah-delhi a class route where the train covers the long distance in a very less time.

---