The 15.6-metre-diameter machines will bore 3.9-km twin tunnels, 40 metres below ground, between Malad's Mindspace and Kandivali's Charkop as part of the Coastal Road North. Mumbai: The Mumbai–Ahmedabad Bullet Train project crossed a major construction milestone on Sunday with the country's largest rail Tunnel Boring Machine (TBM) beginning excavation from Vikhroli.