Document ID: SBB-INF-ZBT-REV-2026
Date: February 23, 2026
Status: Draft for Review
Prepared For: Internal Stakeholders / Federal Transport Office
Subject: Outline and Technical Review of the Zimmerberg Base Tunnel (Phase 1 & 2)
1.0 Executive Summary
This document outlines the technical scope, historical context, and current plans for the Zimmerberg Base Tunnel (ZBT) project in Switzerland. The project is divided into two primary phases. Zimmerberg Base Tunnel Phase 1 (ZBT1) is a 9.4 km operational tunnel that has been a critical part of the Bahn 2000 railway concept since 2003, connecting Zürich and Thalwil .
The core of this review focuses on the planned Zimmerberg Base Tunnel Phase 2 (ZBT2) . Approved by the Swiss parliament in 2019 as part of the expansion step for 2035, ZBT2 is a ~10.8 km new-build project designed to eliminate the current rail bottleneck between Zug and Zürich . The project is currently in the detailed planning phase, with public consultation expected in 2026 and construction slated to begin in 2029 . This review covers the project rationale, technical specifications, construction methodology informed by ZBT1, and the current procurement status.
2.0 Project Rationale and Context
The existing single-track tunnels through the Zimmerberg and Albis mountains present a significant capacity constraint on the crucial Zug–Thalwil–Zürich rail corridor. This limitation prevents the expansion of services and hinders the development of public transport in the Lucerne–Zug–Zürich corridor .
ZBT2 aims to:
- Eliminate the Bottleneck: By providing a modern, high-capacity twin-track alignment, ZBT2 will dramatically increase rail capacity between Zug and Zürich .
- Improve Connectivity: The project will bring the economic areas of Zürich, Zug, Lucerne, and Central Switzerland closer together, reducing travel times and enabling more frequent services .
- Complete the Network: ZBT2 is a key component of the Swiss Federal government’s long-term railway expansion strategy (Ausbauschritt 2035), building upon the foundations laid by the Bahn 2000 and NRLA (Neue Eisenbahn-Alpentransversale) initiatives .
3.0 Technical Project Outline
This section details the technical specifications for both phases of the tunnel.
3.1 Zimmerberg Base Tunnel Phase 1 (ZBT1) – Operational Asset
ZBT1 serves as a critical reference point for the construction of its successor. It demonstrates the viability of tunneling in this specific geological context and highlights the technical challenges that were successfully overcome .
- Route: Zürich to Thalwil .
- Length: 9.4 km .
- Status: Operational since April 2003 .
- Configuration: Double-track tunnel .
- Maximum Speed: 160 km/h .
- Key Construction Challenges & Lessons Learned:
- Variable Geology: The project successfully navigated a transition from stable sandstone (Upper Freshwater Molasse) to challenging, water-bearing loose ground (river gravels) under the city of Zurich .
- Urban Tunneling: With low overburden and sensitive buildings above, innovative methods were employed. These included a continuous grouted ceiling (injection deck), pipe arches, and micro-pile foundation reinforcements to prevent settlement .
- TBM Innovation: A Mixshield Tunnel Boring Machine (TBM) was used, which was converted inside the tunnel from a hard-rock mode to a slurry shield mode to cope with the changing ground conditions .
3.2 Zimmerberg Base Tunnel Phase 2 (ZBT2) – Planned Expansion
ZBT2 represents the next major phase, designed to modern standards for the 2035 horizon and beyond.
- Route: Thalwil (connecting to ZBT1) to Baar (Litti area) .
- Length: Approx. 10.8 km .
- Status: Planning Phase (Vorprojekt completed; public Auflage planned for 2026) .
- Configuration: Two single-track tunnels. This is a significant departure from ZBT1’s double-track design, offering operational flexibility and enhanced safety .
- Timeline:
- Public Consultation (Öffentliche Auflage): 2026 .
- Construction Start: 2029 .
- Estimated Tunnel Construction Duration: Approx. 8 years .
4.0 Review of Phase 2 (ZBT2) Plans
4.1 Technical Approach
The decision to construct two separate single-track tubes aligns with modern safety and operational standards for long rail tunnels. This configuration allows for:
- Cross-passages: Connecting the two tubes at regular intervals for emergency egress and rescue services.
- Operational Resilience: Incidents in one tube do not necessarily close the other.
- Aerodynamics: Better management of pressure waves from high-speed trains.
4.2 Geological and Construction Considerations
Given the experiences from ZBT1, the construction of ZBT2 will face its own geological challenges, particularly in the loose ground approaching the southern portal near Baar. The project plans will likely incorporate a comprehensive geotechnical survey and monitoring regime, mirroring the successful “three-pillar” safety concept used in ZBT1:
- Safe TBM operation (likely slurry or Mixshield for loose ground sections).
- Ground improvement techniques (e.g., grouting, pipe arches) where overburden is low.
- Continuous automated monitoring of surface and subsurface movements .
4.3 Project Management and Procurement
The project is being managed by the Swiss Federal Railways (SBB) and financed through the national Bahninfrastrukturfonds (BIF) . Recent procurement activities indicate significant progress:
- Overall Design and Construction Supervision: Awarded in May 2025 to a joint venture led by ILF Consulting Engineers, with partners including Lombardi AG and Rothpletz Lienhard + Cie AG .
- Specialist Services: A contract for cable planning (Kabelplaner), valued at approximately CHF 1.78 million, was awarded to Gähler und Partner AG in January 2025 .
- General Planning Services: A tender for a General Planner to oversee multiple disciplines (tunnel construction, geotechnics, technical systems, etc.) was active in late 2024, confirming the project’s transition into advanced design stages .
5.0 Conclusion
The Zimmerberg Base Tunnel project is a two-phase endeavor critical to the future of Swiss rail infrastructure. Phase 1 has successfully demonstrated the feasibility and provided invaluable technical lessons for tunneling in the challenging geology between Zurich and Thalwil.
Phase 2, the Zimmerberg Base Tunnel 2, is a well-defined and progressing project. Its technical configuration of twin single-track tubes represents a modern standard, and its planning is advancing through procurement and towards public consultation. The primary risks identified—related to geology and urban construction—are well understood and can be managed using techniques proven during the construction of ZBT1. With a clear mandate, secured funding, and key engineering contracts now in place, ZBT2 is on track for its scheduled 2029 construction start, promising to resolve a critical capacity bottleneck and significantly enhance rail connectivity in Central Switzerland.
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