They said a gangman noticed columns of smoke billowing out of a wagon when the train was passing Kambatham village in Vellore district this afternoon.
He immediately alerted officials in nearby Latheri and Katpadi railway stations, who in turn informed the Fire and Rescue Services Department.
Firefighters fought for two hours to douse the flames after ensuring the blaze did not spread to other wagons, A Annasamy, Katpadi Fire Station Officer, said.
Power supply to overhead cables above the rails was shut down before the fire tenders began their operation.
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On the likely cause of the fire, G Elango, District Fire Officer, Vellore, said it was being looked into.
"It is likely that it was due to self-ignition. Coal has a propensity to self-ignite," he said.
Officials were relieved as loss of coal in a fire would also have meant obstacles in power production in Mettur.