Cochin Express (1967)

Madhava Menon is murdered while on a train journey from Madras (Chennai) to Cochin (Kochi), and jewelry worth Rs. 10,000 is stolen from him. Officer Rajan investigating the crime initially suspects fellow traveler Unnikkannan Nair, but then later clears him and enlists his help in tracing the mysterious third passenger who apparently holds the key to the mystery.