Ystrad Mynach South Signal Box

Ystrad Mynach South Signal Box is a two-storey structure built from rock-faced stone and timber, topped with a hipped corrugated sheet roof that features ventilators. The east trackside elevation showcases two tall round-headed windows with small-paned framed glazing on the lower floor, while the upper floor has a timber gallery with continuous glazing made up of four horizontally sliding sash windows. At a low level against the trackside elevation is a platform that supports two changes in direction for the point rodding and signal wires, directing them to ground level. The upper floor gallery extends to the north and south, and there is a later first-floor extension that includes a partially glazed door leading to the lever room and access steps to the south, likely replacing the original features. The rear (west) elevation is plain stone. Inside, the upper floor retains a block shelf, locking levers, and a track layout plan.