High Quality Content by WIKIPEDIA articles! The Castlereagh Highway is a state highway in New South Wales, Australia, also extending some kilometres into Queensland. It has been given the route number 55. The highway was recently extended past Gilgandra to include state route 86. As such the Castlereagh Highway starts just outside Lithgow at a junction with the Great Western Highway and runs in a north-westerly direction through Ilford and the junctions of Bathurst-Ilford Road and Bylong Valley Way, through the regional centre of Mudgee and Gulgong. From there it joins with the Golden Highway through Dunedoo, then branches off in Dunedoo, continuing past Gilgandra until it reaches the Queensland border via Walgett at Hebel. From there it continues to the outskirts of St George where it terminates with the Carnarvon Highway.