Not just the cracking New Garia station — Metro Rail authorities are equally concerned about the old and decaying underground structure of the north-south corridor. Tracks have subsided in stretches, water is seeping from walls, platforms are damaged, and several other issues have been detected between Tollygunge and Dum Dum on the Blue Line.

Metro officials said once the New Garia station is rebuilt, a major overhaul of the 40-year-old underground stretch will begin. “At present, we are focusing on reconstructing the Up (Dakshineswar-bound) platform at Kavi Subhash (New Garia) station. Once that is done, we will take up the underground stretch overhaul,” said P. Uday Kumar Reddy, general manager, Metro Rail.

He said the tunnel rehabilitation project would cost more than ₹500 crore. Metro has engaged consulting agency RITES to draw up the plan.

Passenger services at New Garia station were suspended on the afternoon of July 28 after cracks were found in the columns of the Up platform. The platform is to be demolished and rebuilt. A tender has been floated, and the demolition is expected to begin before Durga Puja, said Metro officials.

They added that the overhaul of the 16.45km tunnel between Tollygunge in the south and Dum Dum in the north would start once New Garia station is rebuilt. Reddy had said the new station would take nine months to build after demolition.

The entire stretch became operational on September 27, 1995. The first commercial Metro run was between Esplanade and Bhowanipore, on October 24, 1984.

“Periodic maintenance is carried out, but the tunnel now requires a major rehabilitation,” added Reddy.

Key issues identified

Tracks: Metro trains now run at an average speed of 55kmph instead of the optimum 80kmph through the tunnel. “We have been forced to reduce the speed due to the condition of the tracks in certain stretches. Some have subsided,” said a Metro official. “Maintenance is done regularly, but the damage has built up over the years.”

Seepage: Water seepage has been found in several parts of the tunnel. On June 30, services were disrupted after heavy rain caused waterlogging between Central and Chandni Chowk stations. Engineers traced the water to seepage through the diaphragm walls.

Last year, on May 28, Park Street station was flooded after overnight rain during Cyclone Remal, disrupting train services for hours.

Pillars and platforms: Several underground pillars have suffered damage. “There are overhead wires above ground. Stray current from these wires travels through the underground columns, causing corrosion,” said Reddy. “This is inevitable, and we need to repair the pillars thoroughly.”

Platforms have also suffered damage and require overhauling.

Along with tunnel repairs, the signalling system will be upgraded to improve train frequency, said a Metro official.