Exact reason ,I am not aware but from Banglore both KK and Chennai mail are very popular& Prestigious train Rake is operated by SWR.Incoming KK used to arrive by 13.45 to 14.00 and outgoing KK has return trip at 19.00 Hrs . First reason isPM not possible in such less time and where as 12657 comes in morning and it has sufficient time for PM
Next ,by Having RSa even if Incoming Kk is delayed ,12658 can have late departure and won't affect much
This...
more... issue is more severe in Winter as train gets delayed during winter .
BTW ,Banglore division had spare CBC Rake of Kk/mail ,but not sure if it has spare LHB at present .