MUMBAI, May 24 -- In a breakthrough for India's first bullet train corridor, engineers on Saturday lowered a gigantic 350-tonne cutterhead into the ground at Ghansoli's Sawli area, marking a crucial stage in the construction of the Mumbai-Ahmedabad High Speed Rail project. The cutterhead, the massive rotating front portion of a Tunnel Boring Machine (TBM) that cuts through soil and rock, is part of the machine that will dig the project's ambitious 21-km underground tunnel stretch between Ghansoli and Vikhroli. Officials from the National High Speed Rail Corporation Limited (NHSRCL) said the lowering of the 13.6-metre diameter cutterhead completes the primary assembly of the TBM's main shield, making it one of the most significant engineering milestones achieved so far in the bullet train project. The machine is among the largest TBMs ever deployed for a railway project in India and has been specially designed to excavate a single large tunnel capable of carrying both the up and down bullet train lines. The development is crucial because the underground section is considered one of the most technically challenging parts of the 508-km Mumbai-Ahmedabad Bullet Train corridor. Once operational, the high-speed rail line is expected to reduce travel time between Mumbai and Ahmedabad from the current 6-8 hours to just around two hours using Japanese Shinkansen technology. According to NHSRCL officials, the TBM at Ghansoli will begin tunnelling towards Vikhroli from July after final assembly and commissioning trials are completed. Earlier this week, the cutterhead for the first TBM was also lowered at Vikhroli. Officials said both TBMs are expected to begin their tunnelling drives in the first week of July 2026. The enormous machine can operate at a cutterhead speed of up to four rounds per minute and excavate up to 70.56 metres a day under ideal continuous working conditions. While the actual boring distance is around 21 km, officials said the excavation process is expected to take at least a year after accounting for maintenance, repairs and operational pauses. Designed as a mix shield and slurry-based system, the TBMs have been specially engineered to tackle difficult underground conditions beneath the Mumbai Metropolitan Region. According to NHSRCL officials, two TBMs have been procured for the underground section. The first machine weighs around 3,080 tonnes, while the second weighs approximately 3,184 tonnes. Each machine measures 95.32 metres in length and includes major components such as the cutter wheel, main bearing, jaw crusher, erector, main shield, tail shield and four specialised gantries that support excavation work....