Mumbai: City completes rare underground metro crossover

The twin tunnels run parallel to the Western Express Highway. Pic/X/@CMOMAharashtra

In a feat of precise engineering, Mumbai on Thursday successfully completed a rare underground Metro tunnel overlap, with just 1.2 metres separating two major lines. The Mumbai Metro Red Line 7A tunnel passed over the operational Aqua Line 3, marking a milestone in the city's expanding Metro network. The Tunnel Boring Machine (TBM) Disha, working on the Metro 7A corridor, pierced through final segment, completing its underground section.

The breakthrough event was attended by Chief Minister Devendra Fadnavis (in a maroon jacket) and other dignitaries. Pic/by special arrangement

Once complete, Red Lines 9 and 7 will connect seamlessly to Terminal 2 (T2) of Mumbai airport, allowing commuters from Mira-Bhayandar to travel directly to CSMIA. While Disha has completed the Down line, its twin TBM Dhruv, which is working on the Up line, is expected to achieve a breakthrough soon. Currently, Metro Line 7A is 59 per cent complete.

The Mumbai Metro Line 7 corridor extensions are also progressing steadily. Red Line 9 - connecting Dahisar (East) to Mira-Bhayandar - is 96 per cent complete. Phase 1 of this line, the 4.5-km Dahisar-Kashigaon stretch with four stations, is expected to open by December 2025. Phase 2, a 4.6-km stretch from Kashigaon to Mira-Bhayandar with four more stations, will open by December 2026.

Of the total 3.42 km alignment from Gundavali to T2, 2.49 km (in yellow) is underground

Work on the 13.5-km Red Line 7A from Andheri (East) to CSMIA is currently 59 per cent complete. The tunnel is constructed in 1.5 years and the deadline is June 2027.

Tunnel alignment details:

Of the total 3.42 km alignment from Gundavali to T2, 2.49 km is underground. The twin tunnels run parallel to the Western Express Highway, go underground just before the Sahar Elevated Road's vehicular subway, and join the Aqua Line's underground CSMIA Metro station near Terminal T2.

Official comment:

"April 17 marks a monumental milestone as the TBM for Metro Line 7A achieves its breakthrough, completing the first underground tunnel in Mumbai's Metro network by MMRDA. This 1.647-km engineering marvel connects Andheri (East) to CSMIA with a finished diameter of 6.35 metres," said MMRDA Metropolitan Commissioner Dr Sanjay Mukherjee. The breakthrough event was attended by Chief Minister Devendra Fadnavis and other dignitaries.

Key highlights

Project length: 3.42 km (0.94 km elevated, 2.49 km underground)

Stations: Airport Colony (elevated) and CSMIA (underground)

Major challenges overcome: Passing over the operational Aqua Line 3 tunnel, diverting key utilities, tunnelling through shallow depths, and operating beneath the Sahar Elevated Road

Fast track record: TBM Disha was launched 30m underground on Sep 1, 2023, and completed the breakthrough on April 17, 2025