"A probe into the matter has been initiated and if the train driver is found guilty, appropriate legal action would be taken," Divisional Forest Officer of the sanctuary, Ashish, told PTI.
He added that the aged tusker was trying to cross the railway tracks near Khairatia railway station when the incident occurred.
A panel of doctors has been formed to carry out a post-mortem of the elephant's carcass, he said.
Incidentally, the Mailani-Gonda railway track which passes through the Dudhwa Tiger Reserve has led to a number of wildlife casualties in the protected area.