Purulia Express: লালপাহাড়ীর দ‍্যাশে যাবি? চিন্তা কিসের লো? বিকাল বেলা হাওড়া থেকে পুরুল্যা এক্সপেরেস পাবি। - Dip

Good suggestions Sir! I would just like to add that when SER starts getting WAP7, I think, out of the 35 trains you've mentioned, the first 23 should be given priority in getting WAP7.
The big advantage of an induction motor over a DC traction motor is the ability to haul heavy loads at low speeds. For simple conditions though, like for a shatabdi with a small rake and few or no halts en route, while technically induction motors are still better than DC, unless there is any compelling difference between the 2 locomotives in terms of HP, MPS etc, it's not a big advantage.
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For Durontos and Shatabdis, the halts are few ( or none) and the rakes are small (for shatabdi), practically there should be no difference in running with P4 or P7. The additional HOG facility of P7 if implemented should make a difference but from what I've read, it is not good enough to power an entire A/C rake.
We should definitely aim to move towards induction motor based locos, but for now at least, the priority should be given to normal express trains/crawlers/passengers to get P7s. For the high speed segment P4/P7 can be used in tandem. It would be a cruel waste of money and technology if P7's are put to use in a segment where their main advantage over the P4's is not really useful (and 3-phasers are very costly indeed).