Mumbai's overcrowded suburban railway network may soon get a major upgrade as Central Railway (CR) plans the city's first underground railway line. The proposed 7.4 km tunnel between Parel and Chhatrapati Shivaji Maharaj Terminus (CSMT) will form Phase 2 of CR's fifth and sixth line expansion, complementing the ongoing 10.1 km Kurla-Parel stretch (Phase1).

Taking inspiration from Mumbai Metro, CR is considering an underground approach to bypass land acquisition hurdles and displacement issues. A senior official remarked that if the Metro could go underground, there was no reason the railways couldn’t do the same. The line would run 20–25 metres below ground, avoiding surface-level disruptions and weaving around existing infrastructure.

However, the project is still in early stages. Feasibility studies are pending, especially to identify entry and exit points for tunnel boring machines at CSMT. Options include going inside the terminus or using the P D'Mello Road side, though the latter could face alignment issues due to Metro Line 11.

Initially budgeted at ₹890 crore, the project's cost has now spiked to an estimated ₹3,000 crore due to delays and added complexities. Once approved, this underground corridor could revolutionise Mumbai's transit, decongesting its overburdened rail network and setting a precedent for future urban urban rail projects.