The problem for airport service or even local train service is mainly negligence and lack of interest from the SWR authorities. Lack of PF space is not the reason. Even now, there is practically no congestion on the KJM-CSDR-YNK-Chickballapur line and BYPL-Anekal road line. There is no congestion on these routes and thus no obstruction to having a local train service or airport shuttle on these routes. Even PF occupancy in BYPL or KJM or WFD is not a problem for having at least a few commuter trains every day. If the railway authorities' attitude doesn't change, even if other zones trains are not allowed into the city and terminated at the city outskirts, still there won't be any local trains within the city.
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more... Also, it appears that there is a private transport lobby that is not supporting the YNK-Airport branch's construction as private transport from the City to the Airport is a very lucrative business.