The revolver was found by police at Thiruvarur railway station early this morning. The gun belonged to a retired CRPF head constable V Palani.
"The bag was found by 2 AM by us. When we opened it we found a .302 Mark 1 revolver with six bullets loaded in it. It also had 14 more rounds of bullets," Sub Inspector, Thiruvarur Government Railway Police, P Senthamarai told PTI.
He said enquiries revealed that Palani was standing on the footboard of Chennai-bound Kamban Express when he lost balance and fell down near Keevalur between Nagapattinam and Thiruvarur stations late last night.
"The revolver was licenced and it was deposited with District Revenue authorities," the police official said.
Palani sustained head injuries after he fell down and has been hospitalised.
GRP sources said Palani retired in 2009 and was now working as a security guard at Chennai Airport.