The Hastings line is a secondary railway line in Kent and East Sussex, England, linking with the main town of , and London via and . Although primarily carrying passengers, the railway also serves a gypsum mine which is a source of freight traffic. SE Trains operates passenger trains on the line, and it is one of their busiest lines. The railway was constructed by the South Eastern Railway in the early 1850s across the difficult terrain of the High Weald. Supervision of the building of the line was lax, enabling contractors to skimp on the lining of the tunnels. These deficiencies showed up after the railway had opened. Rectifications led to a restricted loading gauge along the line, requiring the use of dedicated rolling stock. Served by steam locomotives from opening until the late 1950s, passenger services were then taken over by a fleet of diesel-electric multiple units built to the line's loading gauge. Diesel locomotives handled freight, also built to fit the loading gauge. The diesel-electric multiple units served on the line until 1986, when the line was electrified and the most severely affected tunnels were reduced from double track to single. Background The South Eastern Railway (SER) completed its main line from London to Dover, Kent in 1844, branching off the rival London, Brighton and South Coast Railway's (LBSC) line at . Construction of a single line branch from (modern spelling "Tonbridge") to Tunbridge Wells, a fashionable town where a chalybeate spring had been discovered in 1606, began in July 1844. At the time, Parliament had not given assent for the railway. The Act of Parliament enabling the construction of the line had its first reading in the House of Commons on 28April 1845. The bill completed its passage through the House of Commons and the House of Lords on 28July, following which Royal Assent was granted on 31July by Queen Victoria. The engineer in charge of the construction was Peter W. Barlow and the contractors were Messrs. Hoof & Son. In April 1845 the SER decided that the branch would be double track. A tunnel was required after leaving Tunbridge. This was named "Somerhill Tunnel" after the nearby mansion. A mile and 54 chains (2.70 km) after leaving Somerhill Tunnel, a viaduct was required. Southborough Viaduct stands high and has 26 arches. A temporary station was built at Tunbridge Wells as the Wells Tunnel was still