Kovai was introduced in 70's- it was superfast intercity on parallel run to Brindavan running to Bangalore and Vaigai to Madurai from Chennai. It used to do a complete round trip in starting days. Later in 1998 the Bangalore-coimbatore ICE came serving as RSA with Kovai as Kovai found it tough to complete round trip and ran late. Till 07 this arrangement continued when Coimbatore Express was extended to Ernakulam. again as per olden days, Kovai does a complete round trip
When Duronto was introduced, it overtook Kovai towards Coimbatore and Kovai started again reporting late runs. Later Duronto timings was revised to see to it it reaches Coimbatore after Kovai