No one was injured in the incident involving Trivandrum- Chennai mail, Railway officials said.
The alert engine driver stopped the locomotive immediately on realising the detachment of the coaches, which came to a halt with their brake system automatically getting activated, they said.
The train, that left here at 2.50 PM, was running at 'normal' speed when the incident occurred between Kochuveli and Kazhakootam stations, about ten kms from here, the officials said.
Thiruvananthapuram Divisional Railway Manager in a tweet said "12624 Chennai Mail detained for 30 minutes at Kochuveli for engine decoupling. It was re-coupled and left at 15.35 hrs".