Malleable iron spike to secure the ends of adjoining iron plate rails on the Laigh Milton Viaduct on the Kilmarnock and Troon Railway. The nail has a square chamfered head which was hammered through a pre-formed chamfered notch in adjoining castings into an oak plug located beneath within a 2 in (51 mm) diameter hole in the centre of a stone block. Each block measured about a foot (0.3 m) square and 9 in (0.23 m) deep. See image for position of nail – centre left and bottom left.
For an article on the viaduct and its restoration see http://www.engineering-timelines.com/scripts/engineeringItem.asp?id=1243.
For a detailed paper on the viaduct and its restoration see here.