Herrenknecht's tunnel renovation machine works around trains that never stop running

Europe's rail network still runs through historic tunnels built between 1850 and 1910 — about 800 in Germany, Austria and Switzerland alone. Their profiles are too narrow for modern trains and safety standards, yet closing each line for renovation is often unbearable for the regions that depend on it. Herrenknecht AG, the German tunnel-boring specialist, answered with the Tunnel Enlargement System (TES), which in April 2025 won the bauma Innovation Prize for machine technology: the first system to renew a rail tunnel fundamentally without stopping train service.

The machine works in stages as it advances along the tunnel on auxiliary rails. Workers first remove one track and relay a single center track, on which trains continue to run. A protective enclosure fully separates the site from passing traffic, so crews can work while trains roll through. Masonry is removed, the surrounding rock is broken out with chisels or controlled blasting, rubble falls to the tunnel floor and is carried away, and a shotcrete robot plus anchors, steel arches and reinforcement mats secure the freshly cut profile until in-situ concrete replaces the temporary lining.

Since January 2024, two TES units have been at work on the Lahntalbahn between Koblenz and Wetzlar, enlarging two tunnels more than 160 years old — the 426-meter Fachinger Tunnel and the 732-meter Cramberger Tunnel — by roughly two meters of radius, bringing them up to dimensions that new tunnels today are built to. Dr.-Ing. Ulf Kirsten of contractor PORR, responsible for the machine technology, called it the first tunnel successfully enlarged this way.

Renovate the tunnel around trains that keep running

Moving the building site forward on rails inside its own protective shell turns a fixed excavation into mobile factory equipment that can serve hundreds of similar tunnels.

The center-track layout keeps one lane of service alive, so the closure cost that normally dominates renovation budgets shrinks to single-track timetable adjustments.

The enclosure is the linchpin: it makes flying rock and dust a machine-internal problem and lets crews work meters from passing trains.

An enlarging machine, unlike a boring machine, reuses the existing bore as its guide and working platform, avoiding new-tunnel alignments, new portals and land acquisition.

The real constraint is not the construction but the closure it forces. A mobile shell that keeps one service lane running turns a project paid in disruption into one paid in machine hours.

The bauma Innovation Prize jury for 2025 singled the TES out in the machine technology category, with juror Johann Sailer of GEDA saying the technology makes it possible to safely renovate and enlarge all rail tunnels during operation. Deutsche Bahn runs a dedicated project portal for the Lahntalbahn renovation, where the TES units have been deployed since January 2024. With roughly 800 comparable tunnels still in service across Germany, Austria and Switzerland, Herrenknecht has turned one pilot into a template for the continent's renovation backlog.