Boyne Viaduct — Drogheda

The River Boyne cut the growing railway into two parts at Drogheda. Before a through bridge existed, passengers had to leave one station, make their way through the town and board again on the opposite bank. A temporary wooden crossing carried trains from May 1853, but the main Dublin–Belfast route needed a permanent structure over the river valley.

Irish engineer Sir John MacNeill designed the viaduct using recent methods developed by William Bindon Blood for calculating the stresses inside large structures. Builders began in 1853 and finished in 1855. Fifteen stone arches approached three iron river spans, producing an eighteen-part crossing about 30 metres above the Boyne.

By the 1930s the middle ironwork needed replacement. The Motherwell Bridge Engineering Company assembled new girders inside the older bridge before taking the original metal away, allowing railway service to continue during the job. The 1932 river spans were narrower and thereafter carried a single track rather than two; later repair programmes treated the masonry and metal across the whole viaduct.

Twelve round stone arches south of the river and three north of it still lead toward the three metal centre spans. Cutwaters, dressed limestone, parapets and plaques bearing the Belfast and Dublin Junction Railway name give the practical structure a monumental face. Trains continue to slow onto its curve between Drogheda and the northern line.

The viaduct closed the last awkward gap in a continuous railway between Ireland's two largest cities. It also displayed a new confidence in calculating and building high mixed stone-and-iron crossings. Its 1930s renewal is important in its own right because engineers replaced the load-bearing centre while preserving both the trains and the great local landmark around their work.