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Balcombe tunnel is a railway tunnel on the Brighton Main Line through the Sussex Weald between Three Bridges and Balcombe. It is 1,141 yards long.
The tunnel was constructed by the London and Brighton Railway during 1840-41. The engineer for the line was John Urpeth Rastrick; the contractor responsible for the brick-lined tunnel is not known.
Ingress of water from the ground above was experienced during the construction of the tunnel, and this has remained a problem throughout its history. Rastrick described the tunnelling as very treacherous, requiring great caution on the part of the miners working it, as "it swells and effloresces as soon as exposed to the air."
Galvanised iron sheets were fitted to prevent the water falling on passengers in open carriages, but the blast from the steam locomotives and air pressure created by the passage of trains could result in the metal sheets being torn from the structure, creating a serious hazard. Thereafter drivers were warned about the hazard presented by hanging icicles.
In July 1903 plans were finalised for the boring of a second Balcombe tunnel as part of the scheme to quadruple the Brighton Main Line throughout, but these were never implemented.
In 2007 catchment trays were fitted under the tunnel's ventilation shafts to divert seeping ground water from the shafts into the track drainage system. The trays consisted of steel decking, gutters and flashings which were installed under limited track possessions