A footbridge crosses the southern approach cutting just in front of the portal which is stone-built with buttresses either side of the entrance. 490 yards in length, the tunnel's lining comprises vertical masonry side walls and a segmental arch in brick. Refuges are provided, many with exposed rock at their rear.
Though straight at the southern end, the bore curves westwards as daylight returns at the north end. The portal there is of a similar design to its sibling.
Traffic over this section of line ceased on 31st March 1963 following a blaze which destroyed Tynycaeau North signal box. The route was officially closed that summer.