City Rail Link Pushes New Digital Engineering Boundaries

The City Rail Link (CRL) project is set to be a game-changer for Auckland’s public transport with 3.45km of underground rail tunnels that will more than double the city’s rail capacity.

As part of the Link Alliance, AECOM with our alliance design partners WSP and Tonkin & Taylor is pushing the boundaries of what’s possible using BIM on linear infrastructure. The team has improved efficiency and reduced risk on the project, and at the same time enabled greater visibility of the overall project design.

Figure 1 – CRL model of station platform and connections

  • Common data environment – the digital engineering team is using a common data environment, to ensure that all 200+ project designers and engineers are working in a single online location with the latest updated designs. The geographically dispersed engineers, across eight different teams, can collaborate and live-share over 50 different models, and simultaneously interrogate and review the current design via a web browser, without any additional software or hardware requirements.
  • New design processes – we employed a combination of traditional Building Information Modelling (BIM) processes, as well as some newer parametric design and generative design processes. This reduced tunnel design modelling time from weeks to hours. When designing the cross-passages (short tunnels that connect two parallel tunnels) in the main tunnels, we used a completely parametric design approach.
  • The power of parametric design – is our ability to regenerate the model based on design changes in just minutes. In addition, the objects in our parametric approach are designed to address different stages of the project, providing the right level of detail at the right time. A cross-passage is a highly complicated element that must be placed based on the physical arrangement of the precast tunnel segments (rotation, tapering, key segment position, etc). Visual programming and computational design have been critical in performing a theoretical calculation of the Tunnel Boring Machine (TBM) ring placement based on an algorithm that aims to minimise the diversion from the design alignment.

Parametric design or modelling is the creation of a digital model based on a series of pre-programmed rules or algorithms known as ‘parameters’. This means that the model, or elements of it, are generated automatically by internal logic arguments rather than being manually manipulated by engineers and/or modellers.


  • Generative design – we have also deployed generative design within the tunnel design to create the ‘kinematic envelope’. This is the space allocated for the safe passage of the moving trains through the tunnel. Engineers used the modelled kinetic envelope to check that the design met the requirements and ensure that no element would be positioned in a way to obstruct the safe passage of the train. This generative design approach saved a considerable amount of time, by reducing several iterations which normally involve interaction with many different disciplines and technical experts.

Generative design is an iterative design process that involves a program that will generate a certain number of outputs that meet predetermined constraints.

Within the tunnel design we have also deployed generative design to create the kinematic envelope. The kinematic envelope is the space that must be allocated for the safe passage of the moving trains through the tunnel.

The project team continues to push new boundaries with the use of digital engineering processes and computational design and is now applying these techniques to other parts of the CRL project to further enhance the design and construction.

About CRL

The CRL is a 3.45km twin-tunnel underground rail link up to 42 metres below Auckland city centre. It includes a redeveloped Mount Eden Station, where the CRL connects with the North Auckland (Western Line) and new underground stations; one mid-town at Wellesley and Victoria Streets provisionally named Aotea, and at Karangahape Road, provisionally named Karangahape.

When open in 2024, CRL will transform the downtown Britomart Transport Centre into a two-way through-station that better connects the Auckland rail network by at least doubling the rail capacity.


For more information, please contact:

Cesare Caoduro