A new vision for the grande dame of Victorian rail

Jay Merrick takes a tour of the elegant concourse to be unveiled at King's Cross

Since the station's entrance was tacked on to it in 1972, King's Cross in London has presented travellers with an oppressive black canopy and a vile scrum-space of a concourse. It may be in keeping with the area's grotty reputation, but it hardly fills passengers with hope or joy. That will soon change, however, with the unveiling of the station's new western concourse.

Designed by John McAslan + Partners, its 140-metre wide canopy is Europe's largest single-span station structure, a fine fusion of architecture and engineering, on the part of Arup, that barely touches the Grade I-listed western façade of the 159-year-old station. This is unquestionably the most innovative piece of British transport architecture since Stansted airport in 1991, and Waterloo's Eurostar terminal in 1993.

The elegant concourse – the key element of Network Rail's £400m modernisation of the station – marks not only its main approach, but the threshold of the £2bn King's Cross Central regeneration zone north of the station, where 67 acres of brownfield land is being redeveloped to create offices, retail and thousands of new homes.

John McAslan gave me a tour of the site. "It's a great democratic space," he said, looking out from the mezzanine balcony at the wave-like glass and steel canopy.

"It's historically democratic, too – something of the 21st century, meeting 21st-century needs, that revitalises a really great piece of 19th-century railway architecture. When the new square in front is completed, the station will be a real public place, not just a terminus."

Standing on the parapet of the station's main façade, above the two vast train-shed vaults designed by Lewis Cubitt in 1852, the relationship between old and new is striking: solar cells crown the train-sheds; a few feet away, a restoration specialist re-points Cubitt's bricks with lime mortar; and, up a steep, narrow staircase in the station's central tower, we enter a musty room with a grimy clock mechanism worthy of Heath Robinson. Below us, in a space the size of a grand Victorian ballroom that will become the new ticket hall, the clunkily ornate original iron wall brackets are being cleaned.

But it's the new concourse that will dominate King's Cross. It is six times bigger than current space and the canopy creates a new 150-metre wide approach to the station that has the feel of an airport terminal.

The concourse design is a triumph of determination over constraints. McAslan and Arup had to find a way to prop the canopy's 1,200-ton glass and steel structure on 17 individual points without any risk to the fabric of the station, or the Underground concourse and lines beneath it.

They've pulled it off, with English Heritage watching closely – and at the same time while consulting 16 stakeholders at one of London's busiest and most complex road, rail and Underground junctions.

Network Rail is to be applauded for commissioning such an adventurous design, a super-crisp brand for the modernisation of the station that will be completed before the start of the 2012 Olympics.

John McAslan: Profile of the architect

John McAslan, 54, is the archetypal can-do architect, generating a shock-wave of endless emails, texts, indecipherably scribbled instructions – and strong design. Until the King's Cross project, his practice was best known for outstanding transformations of important listed buildings such as the De La Warr Pavilion in Bexhill and the London Roundhouse.

Raised in Argyll and the US, he trained at Edinburgh University and set up his practice in 1996 after working for Richard Rogers. He and his original partner, Jamie Troughton, were the first British architects to design a building at Canary Wharf; another early breakthrough project was the Apple headquarters in Stockley Park, Uxbridge. John McAslan + Partners has been British practice of the year four times, most recently in 2009. His team has proved adept in major urban and education projects, and pioneering schemes in Malawi, Haiti, India, and Turkey. McAslan's restoration of the earthquake-shattered Iron Market in Port-au-Prince made him the profession's Indiana Jones.

McAslan, pictured, is famed for not being able to leave things alone, and his pro bono projects have arisen from this trait.

Jay Merrick