The Mumbai-Ahmedabad Bullet Train project has reached a major construction milestone, with underground tunnelling work set to begin in Mumbai even as heavy monsoon rains have forced the postponement of a planned inauguration ceremony.

In a major update for India's first high-speed rail corridor, authorities have decided not to wait for the formal launch and have directed construction teams to immediately begin tunnel-boring operations to keep the ambitious project on schedule.

One of the biggest engineering challenges of the Mumbai-Ahmedabad Bullet Train corridor is the construction of a deep underground tunnel, including a long undersea section beneath the Thane Creek area. With tunnel boring machines (TBMs) now ready for deployment, the project is entering a critical phase that will determine the pace of construction in the Mumbai section.

While Railway Minister Ashwini Vaishnaw was scheduled to formally launch the tunnelling work in Mumbai, persistent rainfall and weather warnings issued by the India Meteorological Department prompted officials to defer the ceremonial event.

However, project authorities have made it clear that the construction schedule will not be affected, with tunnelling activities moving ahead immediately.

Underground work begins in Mumbai

The underground section is being constructed by Afcons Infrastructure Limited as part of the Mumbai-Ahmedabad High-Speed Rail Corridor.

The tunnel extends for 20.37 kilometres between Bandra Kurla Complex (BKC) and Shilphata. It includes one of the most technically challenging segments of the entire bullet train project — a roughly 7-kilometre undersea stretch.

Officials said the first tunnel boring machine will start excavation from Vikhroli towards the BKC station, covering nearly 5.8 kilometres.

A second TBM, currently in the final stages of assembly at Sawli, is expected to become operational within days. Once deployed, it will excavate a 9.7-kilometre section between Sawli and Vikhroli, including the undersea portion of the alignment.

The machines being used for the project rank among the largest TBMs ever deployed for a railway infrastructure project in India.

Each machine features a cutter-head measuring 13.6 metres in diameter, significantly larger than the 5-6 metre TBMs commonly used in metro rail projects across Indian cities.

The customised machines were brought to India earlier this year and have been specially designed for the geological conditions expected along the Mumbai underground alignment.

Construction teams estimate that each TBM can excavate approximately 300 metres every month. Once both machines are operating simultaneously, the pace of tunnelling could reach nearly 600 metres per month.

How much of the tunnel is already complete?

Of the total 20.37-kilometre tunnel length, around 15.4 kilometres will be excavated using tunnel boring machines.

The remaining 4.8-kilometre stretch between Sawli and Shilphata has already been completed using the drill-and-blast technique, marking substantial progress on the underground package.

The tunnel is being built as a single tube designed to carry twin tracks, enabling high-speed trains to operate in both directions.

Supporting infrastructure, including deep construction shafts and multiple equipment rooms, is also being developed as part of the package to ensure operational safety and maintenance access.

Current status of the Mumbai-Ahmedabad Bullet Train project

The 508-kilometre Mumbai-Ahmedabad High-Speed Rail Corridor remains India's flagship bullet train project and is expected to transform rail travel between Maharashtra and Gujarat.

These trains have a design speed of upto 350 km per hour – much faster than the Vande Bharat trains that have a design speed of 180 km/hour.

Large sections of civil construction, viaducts, stations and track-related works have already advanced significantly, particularly in Gujarat. The underground Mumbai segment has long been viewed as one of the most complex portions of the project because of dense urban development, geological challenges and the undersea section.

The commencement of TBM operations is therefore being seen as a crucial step towards completing the Mumbai end of the corridor and moving the project closer to operational readiness.

As tunnelling begins, the focus will now shift to maintaining construction momentum through the monsoon season and accelerating work on one of the most challenging railway engineering projects undertaken in India.

With PTI inputs