The train was speeding between Virar (Palghar) and Mira Road (Thane) where the two on-duty cops -- Constable Chetan Kumar and his in-charge, Assistant Sub-Inspector Tikam Ram -- reportedly had an argument.

The incident occurred around 5.23 a.m. in the B5 coach when Kumar turned his automatic weapon and allegedly killed ASI Ram and three other passengers, including a pantry-car employee.

Later, Kumar pulled the alarm chain, hopped off near Dahisar station and attempted to flee but was caught by the GRP.

He was taken to Mira Road station along with the killer weapon even as top RPF, GRP and other officials rushed to investigate the tragedy.

As per officials, for a few moments, Kumar even tried to hold the passengers hostage at gunpoint, though the exact motives behind his actions are not yet clear.

A forensic team, GRP, RPF and local authorities along with senior officers have started probing the incident.

Meanwhile, the bodies of the victims -- whose identities are being established -- were shifted to Borivali station for further formalities.

The family of the deceased ASI, Ram shall be entitled to various benefits like ex-gratia, Railway Suraksha Kalyan Nidhi, Death Cum Retirement Gratuity, General Insurance Scheme, and other expenses totalling to around Rs 56 lakhs, said an official.