You should remember first that India has zero experience with bullet trains. We have never dealt with highs speed trains of this sort and we do not possess any of the technical know-how in how it is to be implemented and the challenges that can come up during the regular operations. In this context there are a few things to be taken into consideration before constructing a trial line.
Construction costs - A dedicated high speed corridor is a very difficult and costly construction project. Even on a plain, level terrain like BCT-ADI, it it costing about 1 lakh crore rupees. For more difficult terrain like the Deccan plateau (like Delhi-MAS/SBC or CST-HWH) it would cost much more. It would be completely...
more...

pointless to spend lakhs of crores on all these more complex routes even before we become familiar with the technology.

Ease of construction and business - Like I said above, since this is an experimental line, the optimum route should have very easy terrain. Pretty much, only three routes (with important cities on them) satisfy these conditions.
The BCT-ADI route, the Delhi-CNB/ALD route and the east coast route from MAS to HWH. The MAS-HWH route is too long to be a viable experimental line, and that leaves us with the BCT-ADI route and the Delhi-CNB route.
Now the clientele for the Bullet train would be the business class, which would normally prefer...
more...

flight journeys. And this kind of clientele is more abundant on the BCT-ADI sector with a considerable amount of business class traffic.
So, as an experimental line, for a proof-of-concept and also in terms of potential revenue, BCT-ADI would be the best option for a long distance bullet train experiment.