A railway tunnel on the Glasgow South Western Line, the Mossgiel Tunnel is located 1¼ miles (2 km) northwest of Mauchline on the border between South Ayrshire and East Ayrshire Council Areas. Extending to 625.5m (2052 feet) in length, 8.3m (27 feet) wide and 5.8m (19 feet) high, it opened to traffic on the 9th August 1848. The tunnel and its associated air-shaft cut through Permian red sandstone and a microgabbro sill which forms the Mossgiel Ridge and Skeoch Hill. Some of the material removed still lies in spoil heaps above the tunnel. The air shift lies next to the road and descends 48.8-m (160-foot) to the middle of the tunnel.
Water infiltration over much of the length of the tunnel was such that it was collected in a small reservoir near the northern portal and piped to the engine sheds at Hurlford, where it was used to water steam locomotives. This water ingress had removed the mortar between bricks in the tunnel lining and required restorative work in the 1920s. A 426-m / 1398-foot length of the tunnel was relined. A railway halt, Mossgiel Tunnel Platform, once existed just to the north of the tunnel and may have been used by workers in conjunction with this restoration. Tunnel Cottage (now ruined) lies on the opposite side of the road from the air shaft and may also have been built for railway workers.
