Building over stations: urban density drives engineering ingenuity

Rapid population growth, urban densification and increasing pressure on land availability in major cities over the last two decades have been the main drivers for growth in overstation developments.

Sites that were once considered too constrained to develop, such as those above railway stations, tunnels and shafts, are now recognised as valuable opportunities.

This shift has been enabled by substantial advances in geotechnical investigation, appraisal of existing assets, structural analysis techniques, materials and construction methodology.

The nature of the buildings delivered as part of overstation schemes has evolved.

There is now demand for larger, more flexible floorplates with minimal internal columns, particularly in commercial developments. This has driven the use of major transfer structures, long‑span systems and, in some cases, structural facades acting as exoskeletons to redistribute loads to highly constrained support locations below.

In many oversite schemes, the engineering solution fundamentally shapes the architecture, enabling the architectural response and unlocking the site’s full potential.

Geotechnical engineering has become central to this evolution as new techniques have reduced the risk of unknown factors in the ground. Existing foundations can be appraised through advanced analytical modelling, providing assurance on asset protection, particularly where construction takes place over live rail infrastructure.

Challenges such as ground‑borne noise and vibration, settlement control and construction sequencing now routinely inform design, supported by targeted site testing and monitoring.

Designers must also respond directly to construction constraints, often allowing developments many times larger than the original footprint by making use of space above open tracks. This approach sometimes enables designers to achieve up to a fivefold increase in usable development area.

Compared with conventional buildings, overstation developments present a distinct set of design challenges. The substructure is typically governed by existing operational assets, severely limiting column and foundation placement and placing a premium on careful load distribution and interface design.

In London in particular, the presence of operational shallow Underground lines introduces additional complexity through significant ground vibration generated by passing trains. To mitigate the transmission of vibration into the superstructure, isolation strategies are often required.

For example, on the 101 Moorgate project, which Mott MacDonald designed, isolation bearings were introduced above pile caps and at transfer truss level and were carefully positioned to limit bearing size and cost while maintaining performance.

On other buildings where isolation at source is not feasible, isolated upper‑level floor slabs can be introduced to achieve acceptable vibration performance.

Highly constrained sites also frequently restrict the use of conventional tower cranes. This can necessitate alternative or phased construction methodologies or the incorporation of permanent structural elements designed to temporarily accommodate crane loads.

As a result, both the structural design and the construction sequencing become significantly more complex.

To manage this, the tower crane strategy must be considered early in the design process, alongside early development of construction phasing and methodology. Integrating these constraints at concept stage helps de‑risk delivery and ensures the structure is engineered to suit both permanent and temporary requirements.

Construction over live infrastructure demands exceptional levels of protection, planning and assurance. Robust protection decks are typically provided over operational rail assets, often supplemented by hangers from primary trusses and temporary restraint systems to maintain stability during critical stages.

Early thinking on the protection strategy, construction phasing and methodology is also essential.

Doing so helps minimise working hours over live infrastructure, reducing risk and improving buildability. This alignment between design and construction sets a high benchmark for delivery over operational railways.

Mott MacDonald used this approach at the 1 Liverpool Street development, where it supported the successful completion of construction with no reported workplace fatalities or injuries.

The inherent complexity of working above live infrastructure also places a premium on robust programme and design management, alongside close collaboration with key stakeholders. Careful coordination is required to ensure critical milestones are achieved, making effective use of limited access and possession periods such as planned closures and bank holidays.

Digital design tools have been fundamental to the successful delivery of overstation developments.

BIM plays a key role in coordinating multidisciplinary design teams and managing interfaces with existing assets. Detailed 3D models allow designers to understand constraints at an early stage and to develop clearly sequenced construction methodologies. When combined with point‑cloud data and verified asset information, these models provide critical support for construction planning, temporary works design and stakeholder assurance.

Design for manufacture and assembly (DfMA) has also become increasingly important, enabling significant proportions of construction to be undertaken off site. This reduces site duration, improves quality and enhances safety.

At 101 Moorgate, the primary transfer truss was fully welded and fabricated in large sections to minimise site works, while the facade system used precast elements with factory‑installed windows.

Looking ahead, over the next five to 10 years, overstation developments are expected to continue growing in scale and number.

This will be driven by sustained urban population growth, ongoing pressure on land supply and strong policy support for brownfield regeneration and transport‑led development. Public and private infrastructure investment is likely to unlock further opportunities above and around transport hubs.

Key changes in how these schemes are delivered will include greater adoption of DfMA, continued advances in digital engineering tools and earlier integration between developers, designers, contractors and asset owners. Improved identification and reuse of existing foundations, land‑locked piles and redundant infrastructure will further enhance viability.

However, challenges will remain. Upfront costs are likely to stay high, performance requirements will continue to tighten, and regulatory processes may extend programmes and add uncertainty.

Despite this, the direction of travel is clear: overstation developments will remain a critical part of sustainable urban growth, demonstrating that when cities run out of space at ground level, engineering innovation allows them to look up instead.

Åsa Löfstedt is a senior structural engineer at Mott MacDonald.