A CUT ABOVE.

At a singularly forlorn corner in central south London, a new surprising urban space has been created, suffused with sunlight but deep underground.

London, the first modern city, was made by its railways. Awesome expansion in the nineteenth century, which often shocked foreign visitors, was enabled by the seemingly endless extensions of the steel tracks. Since the middle of the twentieth century, the network of metropolitan railways has not been greatly extended, partly because the system was supposed to be adequate, and partly because the dark night of Thatcherism prevented investment in public structures.

Now, a most important thread has been added to the net. The Jubilee Line has been extended to link the north and south of the city across the Thames - and the east of the city which, having been partly destroyed by bombs when it was the biggest port in the world, and then the rapid de-industrialization of the British economy, has now started to have much more life, with new parks, new foundations, new structure which may all perhaps make the great city find new life and energy from the east.

Just as London grew in size with its railways, it grew in presence by their stations. The great terminals: King's Cross, Paddington, St Pancras, were essential landmarks and meeting places of the Victorian city. In the 1930s, Frank Pick and his architect Charles Holden gave the stations of the expanding underground rail system almost equal presence.

The new stations on the extension to the Jubilee Line are intended by its chief architect Roland Paoletti (the Medici of London Transport), to have the same kind of distinction, and, on the whole, his plan has worked triumphantly, [*] with new key moments inserted into some quite unexpected parts of the ancient city. Paoletti did this by the careful selection of architects whom he knew he could trust to make places, but who did not necessarily have a record of working for public transport.

MacCormac Jamieson & Prichard were one of the most surprising choices. Their new buildings for universities are unsurpassed, [**] and have established their reputation as masters of materials, craftsmanship and tender placemaking. Southwark station shows similar sensitivity but on a much greater and more populist scale.

The task was complicated. At one of the scrubbiest junctions in south London, where The Cut meets Blackfriars Road, a new building, 14 storeys of housing, will emerge. Another entrance is from Waterloo East Station, a stop on suburban routes to the south. Both entrances are distinguished.

At the corner of The Cut, the architects have paid homage to Pick and Holden. They have created what they call a 'grand corner', the urban landmark which, in '30s fashion, makes a bold, rounded statement to the cityscape, and at the same time welcomes you in. Down the steps, you come to a generous circular concourse, under a shallow pale dome illuminated by a glazed drum (MacCormac has long been an ardent admirer of Soane).

At the other end of the complex, a glass and steel structure gently reflects on the rather dowdy East Waterloo station with a canopy that complements the existing building and beckons you down into the underground. Organizing the complex must have been very difficult, with many complications caused by the nineteenth-century tracks, and numerous bits of underground urban infrastructure causing a somewhat disjointed plan.

MacCormac realized that the place had to be given a focus, somewhere which unites the two sides and is memorable even in the hurry of subterranean journeys. His masterstroke is the intermediate concourse, which (down escalators) links the two entrance halls. By a fluke of city morphology, this is able to receive light from the sky and is awash with blue light.

One side of the big space is in effect a three-dimensional segment of an oval cone. Here, Alexander Beleschenko, the glass artist, has created a wall of triangular panes of blue enamelled glass, almost pin-striped in pattern; it reflects skylight and, at night, the electric illumination to wash the whole place with cobalt. Opposite the blue curve is a much plainer and more sober plane wall, made of polished grey concrete blocks laid in what I think is called opus pseudisodomum, like ashlar with regular courses but different in height. Roman gravity is echoed in the simplicity of the openings which take you down to the stainless-steel lined passages that connect to the platforms at a further level underground. The whole effect is grand, urban in the best sense, even mysterious, in the reflected blue light.

The most powerful effect of the intermediate space is experienced as you come up from the platforms. The great blue wall becomes a triangulated sky. You come to The Cut with a sense of arrival (awesome if the place is not too busy) that it has never had before. Maybe the area will learn by example to be decent.

(*.) The first completed stations on the Jubilee Line were shown in AR May 1999.

(**.) For instance the chapel at Fitzwilliam College, Cambridge(AR April 1992) and the Ruskin library at Lancaster (AR Jane 1998).

(***.) Beleschenko has worked with MacCormac before to great effect for instance at St John's College Oxford, and at the Ruskin Library.