There was no SWR when this train was introduced. It was an SR train and used to go via Katpadi-Gudur-Vijayawada. There were a lot of trains to Delhi in that route back then and that's why TN was not given any halts. Also Chennai being the most important city in the South back then, required high speed connectivity to Delhi. That was another reason for lack of halts for TN.
Once demand from Bangalore increased. This was given it's own rake and was re-routed via GTL as there was no connectivity to any place in North India for these places. SCR was already giving non-stop run to TN so it was only fair that in return SR...
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allow stops in SCR for this route which did not have any direct trains to North. And besides, this is only a normal super-fast train, so no need for non-stop runs.
This is a very successful train and runs with excellent patronage throughout the year. It is not a good decision to divert this train. Till the Tumkur-Rayadurga route is complete, better to ask for more regular trains in UBL-GTL route so that people in this route can catch connecting trains at GTL. It is a simpler solution than diverting many highly patronized trains into round-about routes.