Rail gauge in Ireland

The track gauge adopted by the mainline railways in Ireland is 5 ft 3 in (1,600 mm). This unusual track gauge is otherwise found only in Australia (where it was introduced by the Irish railway engineer F. W. Sheilds), in the states of Victoria, southern New South Wales (via some extensions of the Victorian rail network) and South Australia, as well as in Brazil.

The Grand Duchy of Baden State Railway used this gauge between 1840 and 1855, as did the Canterbury Provincial Railways in New Zealand, until conversion to the 3 ft 6 in (1,067 mm) gauge in the 1860s. The Launceston and Western Railway in Tasmania also used this gauge from 1871, until conversion to 3 ft 6 in (1,067 mm) gauge in 1888.

Ireland's first railway, the Dublin and Kingstown, was built to 4 ft 8 ¹⁄₂ in (1,435 mm) (later known as standard gauge). The Ulster Railway (UR), taking the Irish Railway Commission's advice, used 6 ft 2 in (1,880 mm). The Dublin and Drogheda Railway was proposed to be built to 5 ft 2 in (1,575 mm) gauge on the grounds of lower costs. The two broader gauges were used nowhere else. Following complaints from the UR the Board of Trade investigated the matter, and in 1843 decreed the use of 5 ft 3 in (1,600 mm).

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