Body shape, coloration (especially lateral yellow stripe) and location all lead me to think this is some species of Uropeltis snake, all of which are endemic to India.
- Wikipedia lists 25 species in this genus, and at least some species are only differentiated by closely examining the number of scales or size of shields.
One common species is Uropeltis eliottii, but I'll admit the coloration looks a tad too dark for your specimen.
Source: The Reptile Database
According to Wikipedia and The Reptile Database, this species is endemic to southern India.
In 2018, a new species, Uropeltis bhupathy was described by Jins et al (2018) from the Anaikatty Hills of the Western Ghats of Tamil Nadu (is this near you?), and I think the scale coloration looks closer to your specimen.
Source: Jins et al (2018)
Jins, V.J., Sampaio, F.L. and Gower, D.J., 2018. A new species of Uropeltis Cuvier, 1829 (Serpentes: Uropeltidae) from the Anaikatty Hills of the Western Ghats of India. Zootaxa, 4415(3), pp.401-422.