(Photos 1, 2, 4, 6 & 7 © K-Burn, photo 3 © MikeBremner,
photo 8 © Jim Barton)
Authorised by an Act of 1870, the Edinburgh Loanhead & Roslin Railway first troubled the timetablers in 1874, becoming part of the North British empire three years later when an extension opened through Glencorse to the fringes of Penicuik.
To overcome the deep valley of Bilston Burn, a viaduct comprising six wrought iron lattice truss spans was erected under the supervision of its designer, Thomas Bouch. But as a result of ground movement triggered by nearby coal workings, it was soon decided to reduce the number of piers from five to two, a project that culminated in 1892 with the opening of a new bridge with three lattice box girders manufactured, erected and riveted together by P & W MacLellan of Glasgow. Its engineers were James Carswell and James Bell. The work greatly benefited from use of the original bridge as falsework.
The central span is huge, measuring 330 feet long by 42 feet deep by 15 feet wide, carrying its single track at a height of 140 feet. The two side spans each extends for 60 feet. Bull-faced sandstone was used for the abutments, close to which were pairs of cast iron roller bearings. These allowed the span to expand by up to 60mm in hot weather. However, after a century in situ, it was found that they had rusted solid and replacements - with a price tag of £147,000 - were installed in 1999 as part of a refurbishment of the viaduct. This involved replacing the deck with a reinforced concrete slab and treating the steelwork with a protective paint system. Costing £1.5 million in total, the work was undertaken by Kvaerner with support from Historic Scotland, Midlothian Council and the Edinburgh Green Belt Trust. It was a precursor to the structure's reopening as a foot and cycle path.
Passenger services over the viaduct ended in 1933 but coal traffic to Roslin Colliery continued to pass over it until 1st June 1969.
(Mike Bremner and Jim Barton's photos, taken from Geograph, are used under this Creative Commons licence.)
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