The rapid transit metro system is designed to reduce traffic congestion in the city and supplement the overcrowded Mumbai Suburban Railway network.

When completed, the core system will comprise sixteen high-capacity metro railway lines, spanning a total of more than 523 kilometres (325 mi) (25% underground, the rest elevated, with a minuscule portion built at-grade) and serviced by 350 stations.

In June 2006, Prime Minister Manmohan Singh laid the foundation stone for the first phase of the Mumbai Metro project, although construction work began in February 2008.

On 5 October 2024, the 12 km underground BKC to Aarey Jogeshwari-Vikhroli Link Road section of Aqua Line was inaugurated.

However, due to the city's geographical constraints and rapid population growth, road and rail infrastructure development has not been able to keep pace with growing demand over the last 4-5 decades.

The main objective of the Mumbai Metro is to provide mass rapid transit services to people within an approach distance of between 1 and 2 kilometres (0.62 and 1.24 mi), and to serve the areas not connected by the existing Suburban Rail network.

However, the MMRDA decided to increase ridership on the line by running it out past Bandra to Chhatrapati Shivaji Maharaj International Airport.

[19] In April 2012, the MMRDA announced plans to grant the Mumbai Metro Rail Company increased management autonomy, in an effort to enhance the project's operational efficiency.

[22] The Mumbai Metro master plan was revised by the MMRDA in 2012, increasing the total length of the proposed network to 160.90 km (99.98 mi).

[26][27][28][29] The revised Mumbai Metro master plan had proposed a line along the Thane-Teen Haath Naka-Kaapurbavdi-Ghodbunder Road route.

The preliminary report proposed a 32 km (20 mi) line with 29 stations, to be built at an estimated cost of ₹22,000 crore.

However, planning for the project will only begun after the construction of the proposed Mumbai Trans Harbour Link commences.

[32] In a report on 14 November 2014 about the cancellation of the PPP agreement for Line 2, Mint quoted a senior MMRDA official: "as decided earlier, all future lines of Mumbai Metro will be constructed by the Mumbai Metro Railway Corp. Ltd (MMRCL), a joint venture between the state government and the Union government.

"[33] On 20 May 2015, Chief Minister Devendra Fadnavis requested officials to consider constructing the Charkop-Bandra-Dahisar and the Wadala-Thane-Kasarvadavali lines as elevated corridors.

[36][37][38] In July 2015, UPS Madan announced that the State Government formally appointed the DMRC to revise and update the Mumbai Metro master plan.

[39] Fadnavis announced on 8 April 2017 that the government was considering a circular metro loop line along the Kalyan-Dombivli-Taloja route.

[40][41] The Mumbai Metro resumed services for general public on 19 October 2020, after being shut down since March 2020 due to the COVID-19 pandemic.

[54] The metro line will connect the Cuffe Parade business district in the south of Mumbai with SEEPZ and Aarey in the north.

[58] A section of the corridor from Aarey Colony to BKC was inaugurated on 5 October 2024 by Prime Minister Narendra Modi.

The line 4 of Mumbai Metro is planned to be a 32.32 km (20.08 mi)[62] long elevated corridor, covering 32[62] stations from Kasarvadavali (beyond Thane) in the north to Wadala in the south.

[53] The Asian Infrastructure Investment Bank has extended a multilateral loan of ₹3,916 crore (equivalent to ₹55 billion or US$634.67 million in 2023)[65] for this project.

It will connect Thane to Bhiwandi and Kalyan in the eastern suburbs, with further extension to Taloja in Navi Mumbai that is line 12.

The line was approved by Chief Minister Devendra Fadnavis on 19 October 2016.The 12.811 km Thane – Bhiwandi section is under construction.

Reliance Infrastructure consulted a number of major international rolling stock builders to provide the train fleet for the Mumbai Metro.

Bidders for the contract included established metro-vehicle manufacturers such as Kawasaki, Alstom, Siemens and Bombardier, but CRRC Nanjing Puzhen of China was ultimately chosen to supply rolling stock for ₹600 crore.

[86] The coaches are fire retardant,[87] air-conditioned and designed to reduce noise and vibration, and will feature both high seating capacity and ample space for standing passengers.

They will be outfitted with a number of features for safety and convenience, including LCD screens, 3D route maps, first-aid kits, wheelchair facilities, fire-fighting equipment and intercom systems permitting communication with the train driver.

[88] The trains will be capable of carrying over 1,100 passengers in a four-car unit, with each carriage being approximately 2.9 metres (9 ft 6 in) wide.

These trains are capable of driverless operations and were built at Alstom's factory in Sri City, Andhra Pradesh.

Capable of driverless operations, the trains are manufactured at BEML's factory in Bengaluru and first trainset for the Yellow Line arrived in Mumbai on 27 January 2021, and will continue to receive the rest of the sets until 2022.