I am not speaking against TN Express starting from Chennai. I just gave an instance that was similar to BZA-NDLS Express.
Speaking on your point,
Sometime back there was some blog comparing Kerala and Tamil Nadu. Whenever a new train is introduced from Tamil Nadu, it is mostly from Chennai to ROI, the biggest beneficiary is the neighboring state.
But...
more... when a train is introduced from Kerala, it is either from TVC or Ernakulam. Eventually, it covers at least half Kerala before exiting the state. Have you ever heard of a train from Palakkad to ROI via Coimbatore. I took the example of Palakkad because it is a divisional HQ.