Concept study for a direct rail link between Aarau and Zurich

Swiss Federal Railways (SBB) is planning a new double-track, approximately 20-miles tunnel link between Rupperswil (Aarau) and Zurich-Altstetten. As a key member of an engineering consortium, EBP was commissioned to carry out a study of the project’s overall feasibility and arrive at an estimate the associated costs.

Current state

As a part of the main east-west transportation axis in northeastern Switzerland, the existing rail line that links Olten, Aarau and Zurich is already operating at or beyond maximum capacity. A consensus has been reached on the need to eliminate the bottleneck in the area of Aarau-Zürich in the medium term. Moreover, the authors of a comprehensive study of development alternatives have concluded that the best solution would be to build a double-track, direct tunnel link between Rupperswil and Zürich-Altstetten.


The alignement of the proposed double-track, approximately 20-miles tunnel passes through geologically and environmentally sensitive areas. Connecting the tunnel to the existing railway network would also be extremely challenging from an engineering standpoint. In light of these factors, SBB commissioned a study of the project’s general feasibility, the presentation of project variants and an estimate of the associated costs (i.e. with an accuracy level of +/- 50 percent). The scope of the assignment also includes a qualitative risk assessment and the proof of forward compatibility with other development projects (e.g. the underground bypass of the Aarau railway station).


The tunnel is to be designed only for passenger trains traveling at a maximum high speed of 155 mph.

Alternative tunnel systems

The scope of the study includes the analysis of various tunnel systems. The variants up for consideration include two single-track tunnels, one double-track tunnel and a combination of both tunnel types. Here, the variants involving shorter open sections would seem to be especially compatible with double-track tunnel systems. Moreover, given that right-side operation prevails on the approach routes from Aarau to Zurich Central Station, provision is to be made within the tunnel link for cross-free changing from right-track operation to left-track operation.

Special challenges

The construction of the new rail line presents many challenges:

  • Optimization
    The tunnel system is to be optimized in terms of cross section dimensions, train speed and energy consumption for each variant using complex ventilation calculations. These optimizations are also to take account of the latest generation of trains.
  • Environmental protection
    EBP is carrying out conflict analyses to identify and demarcate wildlife corridors, nature conservation areas, groundwater flow, and aquifer protection zones. These analyses provide a basis for minimizing environmental impact in the context of the alignement design.
  • Construction phases and logistics
    The concept study also addresses construction phase planning and construction logistics. The integration of the tunnel link into the existing railway network will require detailed studies relating to construction sequences and access to construction sites located between the high frequent railways.
  • Safety
    A tunnel system covering a distance of up to 20 miles will require special safety measures to ensure passenger safety at all times, for instance, in case of fire, technical malfunctioning and train engine failure. The engineering consortium will therefore complete qualitative risk analyses for use in the context of comparing the alternative tunnel systems.
  • Maintenance
    In addition to an assessment of structural feasibility, a maintenance plan will also be drafted for each of the shortlisted tunnel variants. Different tunnel systems may require very different operational and structural maintenance programs, which could also have an impact on the tunnel systems’ basic availability.