The Central Rail Systems Alliance – a partnership between Network Rail, Balfour Beatty, AtkinsRéalis and TSO – has completed a complex track renewal through a narrow Victorian railway tunnel on the West Coast Main Line.
Engineers began working on New Year’s Eve to replace 400m of railway line through the Northchurch Tunnel between Berkhamsted and Tring.
Two fast lines share a tunnel in this location, while the two slow lines, the oldest dating from 1836, have one each.
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Replacement of the track through one of the slow tunnels was logistically tricky because traditional techniques were not an option in such a tight space. Instead, work had to be carried out in stages to get all the old equipment and material out before the new railway could go back in.
The process of the work involved the following stages :
- Track panels had to be carefully removed one by one;
- Bulldozers then pushed old ballast spoil out of each end of the tunnel to be removed by engineering trains;
- A new ballast bed was then created and 18m panels installed within the tunnel using a front-loading (Kirow) crane;
- A special ballast-dropping train then travelled over the temporary rails ('slave rails') dropping ‘top ballast’ before rail-mounted machines were used to profile it;
- A specialist Tamper machine was used to lift the track and shake the ballast below so the track was in the correct position for trains;
- The 18m 'slave rails’ were removed and replaced by longer continuous pieces of rail which have fewer welds, giving a smoother ride for trains and needing less maintenance.
The total work between 31 December and 4 January included:
- 672 new sleepers installed on 24 track panels
- 1,860t of old spoil removed from the tunnel
- 2,200t of new track foundation stone (ballast) laid
- Six engineering trains used
- 430m of steel rail installed
- Staff working a total of 2,440 hours to get the job done
Because of the complexity of the essential upgrade, both slow lines were closed to trains throughout the project, with all trains using the fast lines instead. Lines reopened on 4 January.
