The accident occurred around 75 kilometres (45 miles) from the port city of Chennai around 9:30pm (1600 GMT) when one train slammed into the back of another that was stationary at a signal point.
"Three carriages were derailed by the impact and so far five bodies have been recovered," a local police official told AFP by telephone.
Television channels, which broadcast footage from the site showing rescue teams trying to cut their way into the wreckage, said up to 50 people had been injured.
India's state-run railway system still the main form of long-distance travel despite fierce competition from new private airlines carries 18.5 million people every day.
Experts say the system, the world's second largest under a single management, is desperately in need of new investment to improve safety and help end transport bottlenecks.
The last major accident occurred in July when a packed express train travelling from Kolkata to New Delhi derailed at high speed in the northern state of Uttar Pradesh, killing 63 people.
Copyright AFP (Agence France-Presse), 2011