The train was immediately stopped, they said, adding that there was no damage to life or property.
Preliminary investigation revealed traces of gun powder, phosphorous and sulphur mixed into the shape of a ball and kept on the tracks, they said.
Forensic experts and bomb squad have been pressed into service, they said.
After the blast, security was tightened in and around the station in Tamil Nadu's Coimbatore district in view of a statewide 'rail-roko' agitation by PMK workers over the Cauvery river water sharing issue.
Forty PMK cadres were arrested when they were about to stage a protest at the railway station here.
Since April 1, the state has been witnessing protests by political parties, pro-Tamil outfits, voluntary organisations and student groups, seeking the Centre to set up the Cauvery Management Board in compliance with a Supreme Court order.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)