Great! My favorite loco class. Used to see AJJ WAM's regularly at WFD. It's sad to see them getting decommissioned.
BZA ELS is over-occupied now holding more than 200 locomotives and there is no scope for expansion. Also BZA-BVRT FEDL is in progress and that too has to be considered when planning out for BZA. With GTL-GNT, GTL-Wadi getting electrified, along with the newly electrified GTL-DMM route ( traction substation issues not-withstanding), from SCR's perspective at least, GTL is an ideal location to get a new electric locomotive shed. It's central location also makes it an ideal location for setting up the loco shed.
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By all means SWR should eventually get a ELS, but I feel, at the moment, having only 200-odd km of electrified route, and that too as a single main line with mostly end-to-end traffic and not multiple branches, does not warrant an ELS right away. Definitely once the electrification starts on the TK-ASK route, then SWR should get an ELS. By all means, an ELS is most welcome, but probably not a need of the hour right now.
With RPM shed having expanded with a lot of WAP7's ,ED's fleet is now less burdened and some P4's are being transferred to AJJ ( which still has a lot of scope for expansion) to make more room for locos at RPM. For just the SBC-MYS route, there should not be any problem in providing links once the GTL shed too comes up.

Hardly. GTL will cater to the MAS-CSTM line and also cater to trains heading towards SBC from GR/GNT side. Sheds like GTL, ET, AJJ, ED etc are designed to cater to a wide selection of routes as these stations by themselves do not have much originating traffic and mostly have passing trains. It won;t be surprising if GTL WAP's are seen hanging around SBC in the future.
Again, I am just saying that an ELS is probably not a requirement with only the MYS-SBC route to cater to. Most trains from BWT side get loco links from farther up in their routes like VSKP or ET, or SR links for those originating/passing from MAS. On the GTL...
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side, trains via DHNE, or trains going to DD side will be catered by GTL.
Once SWR see's an expansion in electrification, then an ELS becomes an immediate necessity. For the present, expanding the Trip-shed capacity or making provision at KJM for a handful of locos ( as you pointed out) should be sufficient.