Underground jubilation.

The oldest metro system in the world, London Underground first opened in 1863, with steam trains running along four miles of track between Paddington and Farringdon Street. Now the vast, sprawling network covers 254 miles (over 400km), with 11 lines and 270 stations. The notion of a line serving the south and east parts of London was first proposed in 1949, but it was not until the 1970s that the need for improved connections to Docklands and their hinterlands began to be acted upon. The transformation of the area into a new financial and residential centre helped to provide the necessary impetus and in late 1993, the government finally granted approval for a new route.

The Jubilee Line Extension (JLE) augments the existing Jubilee Line (completed in 1977) which runs from Charing Cross in central London to the north-western suburb of Stanmore. Beginning at Green Park in the West End(1) the new extension will provide a high capacity, fast connection between the Isle of Dogs in Docklands and Stratford to the east. The line will also have wider benefits for Londoners, bringing the Underground to areas of the city that previously lacked an efficient metro link, such as North Greenwich and Bermondsey. South-east London and Kent will be opened up to park and ride commuting, helping to reduce road traffic into the centre of the capital, and international connections will be improved at Stratford and Waterloo (AR September 1993). The first phase of the new line, from Stratford to North Greenwich, is now nearing completion, with the remaining section from North Greenwich to Green Park due to open in the late autumn.

The scale of the JLE operation is staggering. Encompassing eleven station sites, six main tunnel sites, one depot and 18 operating sites, it is the largest construction project in Europe and the most complex tunnelling operation ever carried out below London. Crossing under the River Thames four times, 12.2km of twin-bore tunnels ranging from depths of 15 to 30m have been laboriously hewn under densely built London. The final section of the line (from Canning Town to Stratford) is above ground. Six of the stations on the new route have been purpose designed, with the remaining five radically refurbished and modernised. All vary in their urban settings and relationships with existing transport links. Some are underground - Westminster and North Greenwich, for instance, are huge subterranean caverns, excavated deep into the earth. Others, such as Canning Town and West Ham, resemble suburban outposts with surface platforms and ticket offices.

Coordinating this huge enterprise is Roland Paoletti, the JLE's chief architect and 'Medici of the Underground', who was responsible for commissioning the new stations. In an act of conspicuously enlightened patronage (rare enough in Britain and particularly so for a public transport project), Paoletti selected established, contemporary architects to design nine of the stations(2), with the remaining two (Waterloo and Canada Water) being handled by an in-house JLE team. Beyond ordering certain standard components and materials, such as terrazzo flooring, escalators, glazed platform doors and signage, Paoletti did not attempt to impose specific design guidelines. Instead, his coterie of architects have been encouraged to celebrate the powerful civil engineering forms intrinsic to the project, and as a result the stations share a common language of sleek, hard edged functionalism that synthesises architecture and engineering to generate its own austere elegance. Such precedents were established by Frank Pick and Charles Holden's iconic underground stations of the early 1930s. Inspired by Dutch and Scandinavian Modernism, Pick and Holden's simple geometric massing, uncluttered brickwork and clean lines provided the newly united London Underground with a progressive yet rational corporate image.

Wherever possible, natural light is channelled and reflected into the station concourses, temporarily dispelling the memory of the cramped, dingy conditions that still obtain across much of the existing Underground network. At Bermondsey, for example, designed by Ian Ritchie, the new concourse is bathed in daylight which floods in through the clear glass roof and walls of the ground floor ticket hall. At Southwark (by MacCormac Jamieson Prichard) the station is organised around a dramatic half-conical subterranean hall, four storeys high. Daylight streams down through a large rooflight just above ground level and shimmers through a spectacular curving glass wall made up of triangular glass panels patterned in myriad vivid blue stripes. And at Foster & Partner's Canary Wharf, two delicate, elliptical glass canopies (evolved from the practice's designs for the Bilbao metro - AR May 1997) efficiently diffuse light into the bowels of the station below. At night, the canopies will glow softly, illuminating the surrounding landscape.

Inevitably, the sheer scale and overwhelming complexity of the overall project have given rise to delays and overruns. Originally due for completion in spring 1998, the new line will now be in service by the end of this year. The cost has also risen, from an original budget of [pounds]2.1 billion to current estimates of [pounds]2.76 billion. Some of this shortfall will have to come from London Underground's capital budget, and it has been argued that this will reduce resources for improvements elsewhere on the network. Yet the JLE still represents a considerable achievement, both in terms of engineering prowess and architectural imagination. Its gleaming, airy stations will finally connect south and east London with the city centre, serving to improve the lot of Londoners and setting challenging new standards for the capital's future transport projects.

1 The existing jubilee line from Green Park to Chafing Cross will become a subsidiary spur for occasional use.

2 The full list of JLE architects is as follows: Westminster - Michael Hopkins & Partners; Waterloo - JLE Architects; Southwark - MacCormac Jamieson Prichard: London Bridge Weston Williamson; Bermondsey Ian Ritchie; Canada Water JLE Architects; Canary Wharf - Foster & Partners; North Greenwich - Alsop Lyall & Stormer Architects; Canning Town - John McAslan & Partners; West Ham - van Heyningen & Haward; Stratford - Chris Wilkinson Architects.