Metro services were disrupted for two hours on Monday afternoon after sparks were spotted in the tunnel near Rabindra Sadan station.

A damaged cable is suspected to have caused the sparks, said officials.

The driver of a Dum Dum-bound train that had pulled into Rabindra Sadan station and a few passengers on the platform saw the sparks in the tunnel around 12.30pm, officials said. The sparks were followed by smoke.

The control room was alerted and the passengers of the train were asked to get down. Power supply to the third rail was stopped at 12.45pm. The next 90 minutes saw a frantic search for the source of the snag.

Services between Central and Tollygunge stations were discontinued. Metro officials said trains ran between Noapara and Central in the north and between Tollygunge and New Garia in the south.

Several offices, including those of the state government, were closed on Monday and the passenger count was below average. But hundreds were still stuck at several stations because of lack of proper announcements. “Service available,” read an electronic display board at the entrance to Kalighat station around 1.30pm. People went in to find the opposite.

At least three fire tenders and a police team reached Rabindra Sadan station. A team of engineers and technicians walked on the tracks between Rabindra Sadan and Maidan to find out the location of the spark but failed.

“The spot was finally identified during a test-charging of the third rail. Some sparks flew at a spot in the tunnel, around 50 metres from the Rabindra Sadan platform. A small portion of the insulation of the electric cable was ruptured there,” said a Metro engineer.

The cable is one of those that supply power to the third rail from a sub-station. The copper cable is insulated by a coating of cross-linked polyethylene. An iron mesh covers the structure.

“As the insulation material was damaged, the copper wire came in contact with the iron mesh and a short-circuit triggered the sparks. The circuit breaker tripped power supply as a safety measure,” said the engineer.

The sparks following the test-charging of the third rail led to a flurry of activity. Some Metro employees were seen running with fire extinguishers. “There was nothing to worry after the second round of sparks. It was part of the process to diagnose the source of the snag,” said a senior official.

The damaged cable was isolated from the tracks. After another round of inspection, power supply to the third rail was restored.

“The train that got stranded ran till Park Street without passengers. Following a glitch-free ride, passengers were allowed to board the train at Park Street. Normal services resumed at 2.36pm,” Metro spokesperson Indrani Banerjee said.

The general manager has ordered an inquiry. “The probe will find out what damaged the cable and whether lack of maintenance was one of the reasons for the damage,” said an official.

Till 8pm on Monday, the passenger count was around 3.20 lakh, compared with the daily average of around seven lakh.

Subhankar Chatterjee, 40, had boarded a Dum Dum-bound train at Bansdroni station around 12.55pm. When the train entered Tollygunge around 1pm, the platform meant for Dum Dum-bound passengers already had another train. Chatterjee’s train pulled into the third platform at Tollygunge station.

“The next 10 minutes were spent in utter confusion as no one knew which train would leave first. The announcements were barely audible. On multiple occasions, passengers got off one train and boarded the other, hoping it would leave first. Finally, it was announced that none of the trains would leave as the services have been suspended because of a snag at Rabindra Sadan station,” recounted Chatterjee.