MUMBAI, July 6 -- The Mumbai-Ahmedabad Bullet Train project crossed a construction milestone on Sunday with the country's largest railway Tunnel Boring Machine (TBM) commencing excavation from Vikhroli towards the Bandra Kurla Complex (BKC) station. The official inauguration by Union railway minister Ashwini Vaishnaw was cancelled due to the red alert and heavy rain in Mumbai. The 13.6-metre diameter Mixshield TBM will construct a 6-km single-tube tunnel carrying both the up and down Bullet Train tracks. The tunnel forms part of the 21-km underground section of the Mumbai-Ahmedabad High Speed Rail corridor. Of the 21-km underground stretch, 16 km between Ghansoli's Sawli and BKC will be excavated using TBMs, while the remaining 5 km has already been completed using the New Austrian Tunnelling Method. Railway officials said the TBM, launched from a depth of 56 metres below ground level, has begun its journey towards BKC. Weighing around 3,100 tonnes and measuring 96 metres in length, the machine is among the largest ever deployed for a railway tunnelling project in India. The Mixshield TBM technology is designed to operate safely in mixed soil conditions and areas with high groundwater pressure while minimising ground settlement and surface disturbance. The machine can simultaneously excavate and install precast concrete lining segments, enabling faster and safer construction. The National High Speed Rail Corporation Ltd (NHSRCL) has constructed a 56-metre-deep launch shaft at Vikhroli, along with slurry and water treatment plants, a dedicated power substation, and other support infrastructure. Monitoring instruments, including settlement markers, tilt meters, seismographs and 3D monitoring targets, have been installed to ensure the safety of nearby buildings during excavation. An 11.17-hectare casting yard at Mahape is manufacturing around 77,000 precast concrete segments required for the tunnel lining. Officials said tunnelling between Vikhroli and BKC is expected to be completed by early 2027, while the Vikhroli-Ghansoli section is likely to be completed by March 2028. Separately, suburban train services on Central Railway's Harbour line were affected on Sunday morning after a tree branch fell on an overhead equipment (OHE) wire near Sewri at around 10.25am. Railway authorities imposed a power block on both the up and down lines from 10.45am to 11.20am to remove the obstruction and inspect the overhead infrastructure....