Loco link allocation is very complicated process. WAP5 locos are based only in 2 loco sheds - BRC and GZB. ECR has got only WAP4's, no WAP7's.
WAP7 locos enter ECR because trains like Rajdhanis, SDAH/HWH Duronto, Poorva SF etc get a WAP7 from ER (HWH) or NR (GZB).
Trains like Chennai Rajdhani, Chennai Duronto, NDLS PDY SF, DEE YPR Duronto used to get WAP5 when their link allocation was from GZB shed (NR). Now they get LGD (SCR) WAP7.
NDLS...
more... PDY Exp stopped getting P5 since the LPs at MS weren't trained to handle 3 phase locos. Hence loco change used to take place at MS. Now, it gets a GZB WAP4 regularly.
Let IR decide which loco is to be provided based on availability, working, crew, etc.