A boring machine digging the Metro tunnel from Kidderpore to Victoria, part of the underground section of the long-delayed Joka-BBD Bag corridor (Purple Line), should reach the destination by the first week of July, said railway engineers.

The second machine should finish boring the stretch by the first week of August, marking the completion of the first phase of the underground section of the Metro corridor, they said.

A tunnel boring machine (TBM) (Kidderpore) named Durga started digging the Metro tunnel from Kidderpore to Victoria on July 10, 2025 — kick-starting the construction of the underground section of the Purple Line. TBM Divya began digging the second tunnel from Kidderpore on October 11, 2025.

The two TBMs, which arrived from Germany via Tamil Nadu, were lowered into a launching shaft at St Thomas’ Boys’ School in Kidderpore.

The corridor is now functional on an 8km elevated stretch between Joka and Majerhat. The next stop, Mominpore, will be the last elevated station and Kidderpore the first underground station.

In the first phase, the TBMs are building the twin tunnels to Victoria, a distance of around 1.7km. In the second phase, they will link Victoria and Park Street, a distance of around 900m, said officials of the Railway Vikas Nigam Limited, the implementing agency of the Purple Line.

“Durga has finished boring 95% of the first phase (around 1.65km of the total distance of 1.7km). The machine is expected to achieve a breakthrough by the first week of July. Divya has completed boring 75% (around 1.27km of 1.7km) and is expected to achieve a breakthrough by the first week of August,” said an official.

A “breakthrough” in railway tunnelling marks the moment a TBM breaches the final wall of earth, breaking through into an underground station or a ventilation shaft.

While Durga is boring the BBD Bag-bound tunnel, Divya is boring the Joka-bound tunnel.

Once the machines reach Victoria, they will be retrieved and relaunched to build the stretch up to Park Street, an RVNL official said. The remaining stretch will be built using the cut-and-cover method, they said.

Divya started boring when Durga had 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 a challenging task. Components were brought down in phases, while the remaining parts were retained at the site and assembled later as the machines advanced.

The stretch between Kidderpore and Park Street has multiple heritage buildings. The RVNL official said due precaution was being taken.

“We have already mapped all significant heritage structures and carried out detailed condition surveys. Appropriate instrumentation has also been installed to continuously monitor vibration and noise levels and assess their impact on the buildings,” he said.

The Kidderpore station project remained stuck for years due to the state government's alleged reluctance to allow construction at Alipore Bodyguard Lines. The impasse ended only after a series of negotiations, with the state granting clearance in the last week of June last year.

Construction of the Esplanade station has remained stalled because traders at BC Roy Market have refused to relocate to an alternative site.

Railway officials said they expected the change in government in Bengal to help break the deadlock and pave the way for progress on the station project.