Prior to the opening of the Delhi metro, India's only experience of metro construction had been Kolkata's disastrous rapid transit project, which thanks to political interference, technical problems and bureaucracy took 25 years to implement and was 12 times over budget by the time it finally opened. Delhi by contrast built 65km of world-class metro in the first phase, which was completed two years and nine months ahead of schedule in 2005 at a total cost of $US 2.3bn. By 2010 the network had grown to 190km and a further 120km is under construction as part of Phase 3.

Under the stewardship of its first managing director Dr E Sreedharan, Delhi Metro Rail Corporation (DMRC) drew heavily on the experience of other operators from around the world, notably MTR Corporation in Hong Kong, hired consultants from abroad, and adopted international construction methods and proven technologies to build a safe, efficient, high-capacity transit system as quickly as possible.

Travelling around the city today it is difficult to imagine how Delhi coped without a metro. Ridership has reached around 2 million passengers per day and will double by the time Phase 3 is complete. A fourth phase will take the network to 420km by 2021, when every home in the city will be within 500m of a station.

The speed of development is a crucial factor in the success of system. Delhi is one of the fastest-growing cities in the world - the population increased by 21% between 2001 and 2011 to reach 16.75 million, and without a co-ordinated programme of mass transit investment it is clear traffic congestion would quickly overwhelm the city. In central Delhi around 11% of the land area is required for parking in a city where only 20% of people own a car.

India's urban population is expected to surge from 400 million to 600 million within the next decade and a half – the equivalent of the combined populations of Japan and Germany moving into the country's cities by 2028.

According to India's minister for urban development Mr Kamal Nath, the biggest challenge cities face as they continue to expand is the infrastructure deficit, which can only be met by a $US 1.2 trillion investment over the next 10 years. The government is keen to attract private capital to bridge this gap, and PPPs are gaining traction as a method of financing for a range of major infrastructure projects, including metros.

However, the emergence of PPPs has not been an entirely painless process. A series of reports by the Comptroller and Auditor General (CAG) exposed major flaws in PPP contracting, and the government has recently revised its PPP model to ensure 30-year revenue projections are correctly factored into contracts. In the wake of high-profile failures such as the Delhi-Gurgaon Expressway, the private sector has shown a distinct appetite to move away from 30-year concessions towards engineering procurement and construction (EPC) contracts for major infrastructure projects.

As cities grow into megacities, the government is also studying options for 160km/h regional rail links to better serve peripheral areas and provide fast, frequent services over distances of up to 80km. Again, PPP is the preferred financing option.

If PPP is to play a central role in metro development, any structural problems need to be ironed out quickly, not least because projects that need to move forward quickly face a range of other technical and bureaucratic challenges. In Mumbai for instance, the opening of Line 1 has been delayed by land use and construction issues, while land acquisition problems have also hampered progress on Line 2.

With low car ownership, a young population, and rapidly expanding urban areas, India has a perfect opportunity to shape its cities around mass transit. In Delhi, the metro presents an image of modernity and convenience, a stark contrast with the chaotic, overcrowded, and sometimes dangerous conditions encountered on the suburban rail network. In this sense, metros have the potential to challenge some of the negative perceptions of rail travel in India, and help to keep congestion at a manageable level. But cities will need to build fast if they are to provide sustainable mobility for their fast-growing populations.