This is a peculiar structure, located on Section 1 of the CTRL (now High Speed 1) 19-KM (11.8-miles) north west of Ashford, immediately adjacent to the M20. It resides on that part of the line which formed Contract 420, from Boxley (about 4.8-KM/3-miles northeast of central Maidstone) to Lenham Heath, which was worth £140,000,000 and awarded jointly to ''Hochtief'' and ''Norwest Holst'' in March 1999. Defined strictly as a "cut-and-cover" tunnel, this is a rectangular concrete structure which extends over the double-track line for 150-metres (164-yards).
Four cut-and-cover tunnels were included in Contract 420, mainly because the line passed through a sensitive rural landscape which needed to be preserved. The principle involved digging the railway cutting and then essentially putting a "roof" over the line. Cut-and-cover tunnels were planned for the following locations:
Those tunnels at Hollingbourne and Sandway were built to a curved profile, using precast concrete segments, these subsequently being covered over with layers of earth. Boxley and Harrietsham tunnels were, however, of a different ilk, being constructed with a rectangular cross-section. Whilst that at Boxley was covered over with soil and a grass layer, Harrietsham Tunnel was left completely without any form of disguise, presumably because of its very close proximity to the M20. Its raison d'être appears to be the need to shore up the embankment running along its northern side, in addition to acting as a noise barrier. The tunnels saw regular services from 28th September 2003, when the CTRL's "Section 1" was brought into use for scheduled passenger trains.