My Opinion only:
My suggestions of Locos to the trains
WAP1 = For Jan Shatabdi
WAM1 = For Fast Passenger
WAP 4 = Intercity Express & Double - decker express
WAP...
more... 7 = Rajdhani & Durontos
WCAM Series = For Shatabdi exp
I don't know about diesel Locos. Sorry. This is my suggestion only.