London’s Underground Hero: TBM Caroline Finishes Tunnel Stretch

London just witnessed another major milestone beneath its streets—TBM Caroline has completed a 5-mile underground journey, marking the second successful tunnel breakthrough this year for the HS2 project. Caroline, a 2,050-tonne tunnelling giant named by local school children after astronomer Caroline Herschel, broke through into a foam-filled underground reception at Green Park Way in Ealing. This event brings the Northolt Tunnel—an 8.4-mile twin bore link between West Ruislip and Old Oak Common—to 93% completion.

TBM Caroline was launched in October 2022 and operated non-stop by dedicated teams around the clock. She not only excavated earth but simultaneously installed 4,217 concrete tunnel rings, forming a durable shell for the new high-speed rail route.

To counter high water pressure at the breakthrough site, engineers employed a reception can technique—filling a chamber with foam concrete to balance pressure and safely seal the tunnel before extraction. This process ensures zero water ingress and a controlled TBM retrieval.

TBM Caroline is one of four tunnel boring machines used to construct the Northolt Tunnel. Two machines are carving the western section, including Caroline and her counterpart Sushila, who completed her drive in January. The other two TBMs are now hard at work on the eastern leg from the Victoria Road Crossover Box near Old Oak Common.

HS2 isn’t just a tunnelling triumph—it’s a transformational infrastructure project. As construction continues, HS2 Chief Executive Mark Wild is leading a full programme reset to enhance cost-efficiency and delivery speed. With 93% of the tunnel now complete, the capital inches closer to a more connected, faster transport future.

Check out the following video discussing the major milestone achieved.