First of all let me tell you introducing this train in South India especially Ernakulam, Bangalore, Coimbatore, Trivandrum, Madurai will be waste of time because the flight fares are already cheaper and nearly all the carriers operate their flights at very cheap rate and that too with 180 seater carrier and not with ATR Alliance Air is exception.
Eg. Coimbatore- Chennai All the carriers operate their flights IndiGo, Air India, Jet Airways , Alliance Air.
Coimbatore to Hyderabad by flight is only 45 minutes and same applies Coimbatore-Colombo by Sri Lankan Airlines....
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Already we have lots of trains running between Kerala and Tamil Nadu and special trains too run.
In Rajasthan you can run T18 only in 2020 when electrification completes.