Actually in 1856, the Royapuram - Arakkonam - Walajahpet line was a metre gauge route. It was extended to KPD, then onwards to SA, CBE and then Kerala.
This section is south India's oldest, and also India's oldest with full-fledged stations. Mumbai and Howrah are older, but didn't have proper stations unlike RPM.
VT...
more... was built only in 1888 if I recall correctly.