Poetry in motion.

The modern airport is perhaps beginning to become a recognisable grown-up building type that offers more than just a means of getting onto aeroplanes, but which has a clearly defined series of patterns that give psychological reassurance to passengers as they move through its complex paths between land and air travel and vice versa. It therefore becomes particularly potent when effectively married to that established nineteenth-century type, the railway station, for instance at Roissy (p28), where the might of the TGV (Trains a Grande Vitesse) system - the French end of the splendid new European rail network - is beginning to be linked to the intercontinental air transport system. It could be a portent of the future, for particularly now the Channel Tunnel is working, it may be much easier to take a relatively quick train journey within Europe to Roissy than to catch a short hop flight (with all concomitant hassles) to Heathrow - as yet, still the world's busiest international airport. Foster's Chek Lap Kok in Hong Kong, as can be seen from our short coverage, promises to be even more dramatic experientially, for the trains actually enter the building and their platforms become part of it (look at the transverse section of the terminal on p55): it will doubtless become the international airport for much of south China.

Even without the interaction, airports are acquiring new presence - for instance the huge Denver airport by C.W.Fentress, J.H.Bradburn & Associates (p60), which, for the moment, may be an economic step too far; but it could turn out to be an appropriate symbol for the regeneration of a large chunk of middle America. Hamburg's new air terminal by Meinhard von Gerkan (p56) reinforces the power of the most delightful and successful of north German cities.

Railway stations, too, can help to regenerate, as Arne Henriksen's little station at Sandvika shows (p44). It is a comparatively small intervention which unites two sides of a town previously sundered by the tracks themselves. Even Henriksen's tiny Slependen halt (p48) has a social function in connecting two halves of a suburb divided by a gulch. Much more clearly, the station by Cruz and Ortiz in Huelva is an important device for linking a community (p40).

The importance of transport interchanges has always been celebrated by grand figures in urban life, from the caravanserai and inns of the Middle Ages to the amazing daring of the nineteenth-century railway stations. Santiago Calatrava's wild wings on his station at Lyons (p36) continue this tradition - as does the terminal for containers at Oakland by Jordan Woodman Dobson (p64), where the power of docks is celebrated in a way that does not shame Telford and Brunel - and causes us to ask why so few clients and architects have risen to the romance and wonder of trade. All cities, all communities, no matter how dispersed, need transport interchange buildings. They incorporate human transactions, movements of people, money and perceptions, that require architectural response. That is what this issue is about.

Paul Andreu is nothing if not bold. He has been chief architect and planner of Aeroports de Paris (ADP) for 25 years, right from the creation of Roissy Charles de Gaulle I to the present day. On the way, he has been responsible for airports in the Middle East, Africa, Chile and China, the basic concept for Kansai (AR November 1994) and for the execution of von Spreckelsen's monumental Grande Arche at the end of the Champs-Elysees axis (AR August 1989). Throughout, he has always had a thoroughly French love of grand gesture. The results have not always been entirely happy in human terms. Charles de Gaulle I, for instance, has turned out to be one of the most maddening airport terminals in the world to use. Its annular form round a dreary central void (admittedly crossed with dramatic travelators in transparent tubes) is directionless, extremely confusing and difficult to extend.

Charles de Gaulle II overcame the problems of being too self-centred by being made as a series of separate arcs arranged along both sides of a grand axis. It works well, if a little boringly, and could doubtless be projected everlastingly eastwards in the same pattern. Its major problem was perhaps that each element could only be reached by bus, and this terminal, like the first one, was a good distance from the nearest railway station (again reached by bus).

When the opportunity came to link the airport complex to the TGV (Trains a Grande Vitesse) ring that connects the new fast main lines which focus on Paris, both ADP and TGV welcomed the opportunity with relish. Strategically, it makes Roissy the heart of a huge area that can be reached easily and quickly by train, one that encompasses Brussels and Amsterdam, and could extend to mid Germany, and even to the south of England through the Channel Tunnel. French national planning foresees Roissy as a threat to Frankfurt in the east and the huge London complex of airports in the west. It is a brave and admirable piece of infrastructure planning: the French grand gesture on an international scale.

Tactically, the arrival of the TGV on the site meant that the seemingly unstoppable march of Charles de Gaulle II to the east could be punctuated visually and functionally. Andreu and the architect in charge of French Railways, Jean-Marie Duthilleul, have responded to the challenge with courage and verve. They realised that the RER (Reseau Express Regional - local Paris rail network) must be part of what they call the 'exchange module', and that there was a possibility of creating a local distributor underground rail track that could link all the parts of Charles de Gaulle.

Practicality dictated much of the form. The station had to be below ground, so that aeroplanes could travel overhead to the projected Charles de Gaulle III, which will continue the pattern of the second terminal to the east. The tracks could be brought in at right angles to the axis of the terminals, so obviating the economic and structural problems of squint bridges over a railway. (Compared to the problems faced by Nicholas Grimshaw and Anthony Hunt in bringing the passenger terminus of the Channel Tunnel terminus to a tight twisted site in central London - AR September 1993 - the Module d'Echanges is a simple Cartesian diagram.)

But this does not mean that the Module lacks imagination and joy. Andreu was determined not to make the place into a cave or tunnel (a form common to almost all railway stations attached to airports, even the magnificently experientially successful Stansted - AR May 1991). If a cutting had to be made for the rail tracks, it did not need to be sealed over, and copious daylight could be brought right down to platform level. The architects turned to Peter Rice to make a glass enclosure which would give an appropriate sense of movement within enclosure.

At the same time, Andreu wanted to emphasise the crossing of rail, air and road traffic, so he has created a lozenge-shaped hotel (still building) rising over the point of junction. The hotel itself promises to be a rather dull, rationalist affair: in effect, the positive form of the negative spaces along the axis of terminal II. Roads ramp up to the entrance of the hotel and passenger set-down from surface transport. Below this level is a floor for air terminal related sevices, then going on down, there is the mezzanine for the tracks of the Charles de Gaulle internal rail transit system, then the TGV/RER station and ticketing hall, and finally the rail platforms themselves at the bottom of the trench. Andreu has permeated this nexus of traffic routes by a series of shafts of light that fall through the whole complex. (There is even a central well in the middle of the hotel through which you will be able to look right down to the platforms - we shall see how this rather unpromising narrow device works in a few months).

But the route upwards from the platforms is magnificent. Rice's brilliance is still being revealed to us in works that are being completed two years after his tragically early death. He could speak in so many architectural languages and make poetry of them all. At Kansai he and Renzo Piano made a compelling blend of the organic and industrial technology. At Roissy, in creative interaction with Andreu, he was much more Cartesian - but his great humane mastery of subtle and complex geometry is quite clear.

The section of the flat glass roofs which extend over the lengths of the platforms, to north and south of the node, curves transversally to the tracks to shed water on either side. It is supported on relatively simple vertical and diagonal props. But the glass roof must slope up from this level towards that of the roads that serve the entrance floor of the hotel. As it rises, the transverse glass curve gradually flattens to become an inclined plane: all with standard glass elements. This is a remarkable enough achievement in itself (and one that allowed the complex form to be built economically).

Yet Andreu and Rice had a far more complex existential and structural programme than simply juggling curves. They wanted passengers to get involved with the materiality of the building as they were drawn up through it by the escalators. From being merely enclosed by a vault of glass on the platforms, you rise and are gradually enclosed - and involved with - the tactility of elements of structure. Andreu hoped to make the lower parts of the structure heavy, tactile, imminent. Above, he wanted the glass skin to float in increasingly ethereal insubstantiality. Rice responded by making inverted bow-string trusses in which the tubular bottom boom curves down towards the escalators and the tensile stresses are taken by thin members which hold the ends of the compression boom together and then transfer upward load to the ground.

The skin itself is supported on thin struts propped off trusses. The booms are supported on columns that fan out along the line of the tracks from stainless steel nodes on top of pylons that take compression loads downwards. They cantilever out beyond the column rows to give the building presence as it rises above the ground.

The glass walls and the roof of the station do not meet. This is not an air-conditioned space, but one in which temperature must be controlled passively. So the glass is fritted to prevent excessive heat from the sun. But Andreu's specification for the fritting has unusual elegance: 80 per cent of solar radiance is cut out by the topmost glass panels and only 20 per cent by those at the edges, so during the day there is an inverted (but sensually appropriate) sense of zenith and horizon.

Andreu had another reason for wanting to use the fritting. At night, a clear glass vault in a cutting could have felt almost grave-like. Yet, with uplighters, the fritted glass becomes a shimmering canopy over the whole dramatic progression from platform to higher levels. The ethereal effects, both at day and night, are greatly enhanced by Andreu's insistence that every piece of structure be painted white: as the members rise and become slender, they begin to be visually assumed into a dreamy insubstantial upper layer between sky and reassuringly stable earth-bound structure. Not a bad introduction to the magical experiences of flying.