You are talking generally, so, you are right.
But, many things are there that matter.
For CSTM-Howrah trains.... Santragachi (Howrah) SER is not prepared for 3phase (WAP-7 and WAP-5) maintainence.
precise reason is that Sevagram is 24 coaches and WAP-7 that hauls it also belongs to Nagpur shed Ajni. Hence, Sevagram gets a WAP-7 and also Nagpur Duronto gets WAP-7.
A...
more... loco is given to a particular shed n zone to serve thier respective trains
Like Ajni shed WAP-7 hauls trains that start from Nagpur towards Pune, New Delhi and Mumbai.
This Duronto serves Mumbai and Howrah people and takes just technical halt at Nagpur
While Bilaspur Rajdhani halts at Nagpur and allows passengers to board and deboard and also, despite being of NR zone, its after all a rajdhani, hence, it gets CR AJNI WAP-7.
Again, Chhattisgarh Express being long train like sevagram, it gets AJNI WAP-7
Once, Kalyan shed gets WAP-7, then may be Howrah duronto might be given Kalyan WAP-7 but Santragachi ELS shud be prepared for 3phase maintainance