Bholanath Mahato, an active member of Peoples' Committee Against Police Atrocities (PCAPA), was arrested from Khemasuli village near Jhargram in West Midnapore district, the sources said.
Mahato was allegedly involved in cutting open the pandrol clips (used to fix the rail to the sleeper) which resulted in the train's derailment.
The Mumbai-bound Gyaneshwari Express derailed May 28 between Sardiha and Khemasuli stations and subsequently a freight train approaching from the opposite direction rammed into five derailed coaches, killing 148 people.
Following the incident, the state police's Criminal Investigation Department (CID) initiated an inquiry, but later handed over the case to the CBI June 10.
While the CID nabbed two people -- Khagen Mahato and Samir Mahato, the CBI had arrested one suspect, Hiralal Mahato, for his alleged involvement in the case. But all the prime accused are still evading the arrest.
The CBI Thursday declared a reward of Rs.1 lakh each for anyone providing information about the three prime accused-- Umakant Mahato of Banksole village, Manoj alias Bapi Mahato of Rasua village and Asit Mahato of Krishnanagar village in West Midnapore district.