Sir, nobody here is aware of any solid agreement between SCR and SWR of loco sharing, am skeptical of existence of any such thing as far as electric loco and EMD loco sharing is concerned. Points in Favor:
1. UBL was in SCR when it started recieving EMD's, SWR did not exist that time. So factually SCR lost all of its EMD's to SWR after zone bifurcation.
2. Even if RPM WAP-7 increase in number slowly, they are not too high yet to act as a link for long distance SR trains and...
more... Short distance trains alike,
3. SWR uses many GY EMD's and ALCO's in its Braganzha and subhramanya ghats section even till date.
I dont think SWR EMD P-4's will therefore cease to haul SCR ttrains even after RPM gets WAP-7's.