An electrical spark on the Metro Railway tracks disrupted services for 40 minutes on Monday afternoon.

The driver of a Tollygunge-bound train saw the spark while the train was approaching Chandni Chawk station around 1.25pm. He alerted senior officials.

“Sparks are common at the points where the train or the feeder cables touch the third rail. They are harmless. On Monday, the driver felt the magnitude of the spark could not be ignored and alerted us,” said an official.

A team of engineers and senior officials rushed to the spot around 2pm. Services were suspended between Park Street and Mahatma Gandhi Road. Trains continued to ply from Tollygunge to Maidan and Dum Dum to Girish Park. Two pairs of trains were delayed.

“We suspended the services as a precautionary measure. The power supply to the third rail between Park Street and Mahatma Gandhi Road was cut off for inspection. After our engineers gave the go-ahead, full services were restored,” said Pranab Dutta, a senior public relations officer of Metro.

An engineer said the spark was probably caused by a fault in a cable that supplies electricity to the third rail.

“The engineers found a minor fault in a cable and repaired it. We will conduct a thorough investigation to find out whether there are other faulty cables tonight,” said Dutta.

According to him, no other driver reported any such spark along the route.

“We will, however, carry out a special inspection of the trains, tracks and the third rail,” he added.