A tunnel boring machine (TBM) began digging the second tunnel from Kidderpore on Saturday evening, speeding up work on the underground section of the long-delayed Joka-BBD Bag Metro corridor (Purple Line).

Two TBMs had earlier been lowered into a launching shaft at St Thomas’ Boys’ School in Kidderpore. The first machine, Durga, began digging the BBD Bag-bound tunnel on July 10. On Saturday at 6.23pm, Divya, the second TBM, was launched to bore the Joka-bound tunnel. The launch was flagged off by Subhransu Sekhar Mishra, the new Metro general manager, amid loud cheers from workers at the site.

An official of Rail Vikas Nigam Limited (RVNL), the implementing agency for the Purple Line, said the two TBMs are part of a push to finish tunnelling between Kidderpore and Park Street. The corridor is currently operational on an 8km elevated stretch between Joka and Majerhat.

The launching shaft, built on the school campus, is 37 metres long, 22 metres wide, and 17 metres deep. Each TBM is around 100 metres long, weighs about 600 tonnes, and has an outer diameter of 6.63 metres. The tunnel is being constructed using precast concrete segments.

RVNL’s chief project manager for the Purple Line, Vipin Kumar, said Durga has already completed around 150 metres of tunnelling. The two machines could not be launched simultaneously due to limited space at the site.

Lowering both machines within a month was challenging. Parts of the machines were lowered in phases, and the remaining components were kept at the site to be connected later as the machines moved forward.

Kumar explained that a larger, 65-metre shaft would have required a traffic diversion on Diamond Harbour Road — something they wanted to avoid. As a result, the current shaft accommodates only 12 metres of a TBM, with the rest staged outside.

“When the first TBM had bored 100 metres, we paused for over a month to lower the remaining parts and reconnect the umbilical cords to the second TBM. It was a very complex operation,” Kumar said.

The next station after Majerhat will be Mominpore — the last elevated station. Kidderpore will be the first underground station. In the first phase, the TBMs will dig to Victoria station (around 1.7km away), followed by the Victoria-Park Street stretch (approximately 900 metres) in the second phase.

Once the machines reach Victoria, they will be extracted and relaunched to build the remaining section till Park Street.

“We expect the first TBM to reach Victoria by June 2026, and the second by December 2026. Both should reach Park Street by March 2027,” Kumar said.

The TBMs were shipped from Germany and arrived via Tamil Nadu.

The project had faced major delays, particularly over construction of the Kidderpore station. One key hurdle was the state government’s refusal to permit work at the Alipore Bodyguard Lines. The deadlock ended in late June following multiple rounds of negotiation.