The main reason behind very rare ED loco or RPM loco is because there are limited number of trains from Tamil Nadu to North destinations when compared to the Andhra Pradesh /Telangana and if at all Kerala have trains to North India like Delhi,Chandigarh,Amritsar,Dehradun,Mumbai,Ahmedaba d or so they can use only ERS WDM-3 or GOC WDP-3/4/4D upto BRC and then BRC P4/5 takes care.(via Konkan)
For many trains RPM/ED goes straight and return back but sometimes HWH trains come with SRC or HWH locos in Chennai mostly.