RB has sanctioned a lot of premium trains on Friday for other zones 12776/12740/22207/20601, so I hope you are again specifically addressing SWR in this regard.
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There was this tinkering with 12735/12736 and Friday Bangalore departure was swapped to Saturday with awful timings(which was later reworked), so I smelled conspiracy at the time but that was an SCR train and the new schedule has benefited the Hyderabad passengers and the patronage which was disrupted rose again. Moreover around the same time the Bangalore Rajdhani timings got standardized.
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As for 12557, the train was introduced with this itinerary which has remained unchanged. The reason for its Thursday run is simply because of another train 12555 that shared its slot on Fridays from YPR to SA. The Pondicherry crowd did not take well to the GR like the Keralaites and in a few years the train was scrapped. Even now, its successor 16573 runs with a similar timing. So I don't see SWR actually doing anything here except continuing what it previously did rather than single out Kerala passengers or something. SR could have easily launched a Tri-weekly in the opposite schedule, but it was not done until now. They introduced this Humsafar only after they failed to replace 12611 for higher fares, with the same rake.