A Calcutta Metro train should not be able to start if the gap between the doors is more than 15mm. On Saturday, the train ran almost 60 metres into the tunnel after a passenger’s hand got stuck in the door.

This suggests a failure of the so-called “fail-safe technology” that is meant to prevent accidents like the one that killed Sajal Kumar Kanjilal, 66. A probe will ascertain if human error contributed to the tragedy.

The doors in all Metro coaches have a permissible gap, called “tolerance limit” in railway parlance. The limit allows a train to run despite a small gap — 10mm to 15mm, according to officials — between the doors.

If the gap in any door is beyond this limit, power should not reach the engine and the train should not be able to start, an official said. That the train started moving on Saturday indicates a malfunction, Metro sources said.

“If we have a zero tolerance limit, a train will stop even in case a well-built adult leans on a door. It will then be impossible to run crowded trains,” an official said.

Often, though, the older trains are found to run even with hair clips and other small objects thicker than 15mm stuck between the doors in a packed compartment. In the new air-conditioned rakes — like the one Kanjilal was trying to board — the driver’s cabin has a panel with a monitor that shows “open” and “closed” icons for the doors in all eight coaches.

Had the technology worked, the train should not have started since a panel would be flashing an “open” door. A few engineers called it a “circuit failure”.

A probe will also ascertain whether the driver had ignored the signal on his screen.

The new rakes have talkback units through which passengers can speak to the driver with the help of a buzzer. But at least two passengers said they were unable to do so. It is not clear whether the devices did not work or whether the passengers did not know how to use them.

Prima facie, sources said, it appears the driver and the guard did not abide by standard operating procedure. The guard is supposed to look out of his cabin at the rear to see if all the doors have closed. He presses a bell twice to signal an all-clear to the driver in the front cabin, who is supposed to reciprocate by ringing his bell twice before starting the train.

Many passengers running to catch a train after the bell rings throw a bag or put a hand or foot in to stop the door and force the guard to reopen it. In this case, the guard did not notice that Kanjilal’s hand had got stuck.

The accident would have been averted had the guard spotted this.

“When trains and platforms are too crowded, the guards sometimes give the all-clear signal without checking whether the last door has closed,” an official said.

But sometimes the guard may have already checked all the doors and withdrawn into his cabin when a passenger makes a last-moment dash.

On Saturday evening, the train was crowded; so CCTV footage of the platform will be crucial to deciding whether the SOP was violated.

The routine maintenance that new trains are required to undergo includes checks to see if the train moves despite people putting their hands or any objects between the doors.

Since Metro is saddled with an ageing and snag-prone fleet, the maintenance of the new trains may have taken a backseat, a railway official said. Some of the older trains are as old as the Metro itself and their maintenance eats up time and manpower.

The trains on the Delhi Metro have obstruction sensors that prevent the doors from shutting when they sense an obstruction; and if any door does not shut, the train cannot move, two senior Metro officials told The Telegraph.

“This obstruction-sensing mechanism is highly reliable,” one of them said. But obstruction sensors are machinery, and malfunctions can occur, he added.

The officials were not immediately able to say at what threshold degree of obstruction the doors are programmed not to close. This would likely depend on the actual mechanisms underlying the sensors.

For example, some types of sensors may respond differently to thin or soft materials than to obstructions that pose greater resistance.

A PTI report in April this year had described how a 40-year-old woman had suffered head injuries after her sari got stuck in the carriage door of a Delhi Metro train and she was dragged a few metres as it was about to leave a station.

Another commuter had pressed the emergency button to get the driver to stop the train.