The accused was arrested this morning from Mullanthurthy in Ernakulam district, police said.
On August 25, Senthil Kumaran, picked up from a house in Shornur in Palakkad district, told police that he had planted the "lunch box bomb" with the motive of causing trouble to a rival, identified as Thomas.
He had engraved the name of Thomas on the steel box found on the track near a rail bridge. It contained a metal plate on which the the name and number of his rival's motorcycle were also written, police said.
Thomas and Senthil, both bus drivers, were friends earlier but had a tiff sometime back over some personal issue.
The lunch box containing explosive chemicals, timer, batteries and fuse wire was removed from the bridge across Muvvattupuzha river near Piravom Road station on Thursday, they said.
