Rightly said bro, that is why even the route MS-MDU-Tirunelveli-Nagercoil-TVC is shorter than the one via Salem, not many trains run on the route.
For Shencottai route, we can expect that atleast 536 km MS-Virudhu nagar (out of total 760km of MS-QLN) shall be double electrified line within a few years. Rest of the route shall be single line (224) with low traffic and ghat section.
Still we can expect a crack train on the route to take 4-4.5 hrs for these 224 km(Virudhunagar-Kollam) and 8 -8.5 hrs for MS-Virudhunagar...
more... 536km and giving a jouney time of under 13 hrs which is good enough as compared to MAS-TVC mail.