Bridging the Tracks: Air Rights Development and the Urban Fabric

Broadgate – Exchange House, London © SOM | Alan Williams


Essay by William F. Baker and Richard F. Tomlinson II


Riverside Plaza. The Broadgate Development. Millennium Park. These developments, each built atop a complex network of rail yards, have forever changed the urban landscape of the cities in which they inhabit. While rail yards can often fragment a city, dividing it into unsightly zones that are difficult to cross, air rights developments can re-stitch the urban fabric, adding usable open space and potential amenities. Whether the rail right of way is privately owned (as in the United States) or publicly owned (as in France), these developments create value for owners, developers, government entities, and private citizens.

The disposition of railway tracks and the presence of the railway station have always been key elements of the socio-economic geography of cities. Invariably, railway stations and railway-owned land are at the hub of urban activity, ideally located in terms of public transport, infrastructure and accessibility. While the station has traditionally been celebrated as an arrival point of major urban significance, the presence of the tracks and the noise and pollution generated by moving trains have always interfered with the continuity and quality of urban life. As a response, the development of air rights has been an established practice for many decades. The premise is simple: build in the unused space above and around railway tracks and stations, in order to provide the opportunity for commercial development and enhanced public amenities. While the character of each development is dependent on the particular conditions of the site, air rights developments can be broadly divided into three basic types: 1) over the station terminus building; 2) over station platforms; and, 3) over through-tracks outside stations. While all three promote connectivity from one area of the city to the other, the first two create density around existing high-volume transit locations. From a developer’s perspective, this presents an ideal situation, as access to rail traffic creates real estate value and opportunity. These high traffic areas are easily accessible and appealing to commercial tenants seeking to provide ample commuting arrangements for their employees.

The following case studies prove that commercial development has been successfully completed over rail facilities without major relocation or disruption of operation. In doing so, designers have produced additional benefits for the railway, while providing state-of-the art office and commercial facilities.


Riverside Plaza at Union Station, Chicago

The Riverside Plaza, a series of buildings and public spaces built over the Union Station terminal in Chicago in the 1960s and 1970s, is a pioneering example of air rights development that enhances the public realm with informal riverside seating and landscaped pedestrian routes. Located along the Chicago River at the location of the original Union Station, Riverside Plaza 1, 2, and 3 are built over the Amtrak and Metra Commuter Rail system lines. These buildings bridge the gap between the Loop and near west side, and were successful in shifting the commercial axis of the city westwards, bringing new life to a previously derelict industrial area.

The construction of the first two 850,000 sf buildings at Riverside Plaza was accomplished while maintaining rail traffic handling 85,000 daily commuters and intercity passenger trains at the Chicago Union Station. Both towers are carried on 48 columns and caissons, with long bays spanning the tracks at 18’ x 45’ wide intervals. Their deep “rafts”, the horizontal planes that span the tracks to form a ‘ceiling’ and act as the ‘ground level’ for the new buildings above, allow for exhaust plenums and services distribution for each of the 50,000 ton buildings. Designing over an active terminal provided engineers with a unique advantage: an established set of platforms that could act as the location for building foundations. Because the station’s platforms run in parallel, they provide a regular interval at which to drop columns. Engineering becomes increasingly complicated when the site is located above track configurations outside of the platform area, as is the case with Broadgate Exchange House.


Gateway Center I and II under construction. © SOM

Gateway Center I and II. © SOM | Hedrich Blessing


Broadgate – Exchange House, London

Completed in 1990, Exchange House is an integral part of the overall Broadgate development, creating both a physical and a visual gateway to the Exchange Square plaza and the Liverpool Street Station beyond. The construction of London’s Broadgate Exchange House catalyzed the development of new streetscapes, lawns, bridges, and plazas surrounding the site. It amounted to one of the largest net additions of new infrastructure in one of the world’s most densely developed cities, and exemplified the urban planning principle that virtually all of the world’s cities can generate essential new space by utilizing air rights in tandem with sophisticated engineering techniques.

Broadgate is a building-bridge hybrid; its form and structure are built on an exposed steel bridge spanning a 78-meter-wide group of tracks and switches that bring the trains from the long distant main line tracks to the individual platforms in the station. Because the structure was to reside over a track configuration that did not permit adequate bearing points, the entire width of the tracks needed to be bridged with a spanning structure that would carry much of the building’s floor loads directly to the foundation below. The resulting building arches over the train tracks, enabling the Liverpool Street Station to function normally underneath a 10-story office building.


Broadgate under construction © SOM

Broadgate under completed © SOM | Alan Delaney


Millennium Park, Chicago

In June 2004, the City of Chicago fulfilled a decades-old dream by creating a major 25-acre park above an existing unsightly rail yard and surface parking lots along Michigan Avenue adjacent to Grant Park, while instantaneously creating a significant tourist destination as well as cultural and recreational facilities for the general public. As Chicago’s most ambitious outdoor cultural project since the Columbian Exposition, Millennium Park forever changed the city’s landscape, providing access to the lakefront and shifting public perception of the urban park.

Millennium Park is supported by a massive bridge-like structure constructed with steel and pre-cast concrete spanning up to 120 feet and is the foundation for the more visible parkland areas above. It is built over existing and expanded rail lines, a busway, and two multi-level parking structures that unite in forming a multi-modal transit center. While many of the railway tracks below the park were abandoned, engineers had to design around the active railway tracks of the South Shore Line, whose service extends into Millennium Station. As is often the case with air rights development, particularly complicated development areas yield open space; as such, no buildings were built above the South Shore tracks.


Millennium Park before construction © SOM | Peter Barreras

Millennium Park Master Plan completed © SOM


With the establishment of high-speed inter-city trains throughout Europe and Asia, the railway station is once again becoming a major entry point for international and regional passengers in many cities. The advent of 21st century rail service will catapult the importance of air rights to an entirely new level. With its “built-in” destination for thousands of passengers and commuters on a daily basis, railway land constitutes a prime development opportunity and extends the traditional catchment area beyond its immediate surrounds to a regional and international level.


Millennium Park Master Plan © SOM | James Steinkamp Steinkamp Photography


William F. Baker is the structural engineering partner at SOM. Throughout his distinguished career, Bill has dedicated himself to structural innovation. His best known contribution has been to develop the “buttressed core” structural system for the Burj Khalifa.

Richard F. Tomlinson II is a managing partner of the SOM Chicago office. Throughout his tenure at SOM, Mr. Tomlinson has managed projects that span industries and continents. He has experience in large scale mixed-use, corporate, residential, hospitality, financial, law, healthcare, and research architecture.

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