Opened : 1851

Closed - 1964

Engineering Code: VON

Length: 703 yds

Cefn-Glas Tunnel was built to take the extension of the GWR route from Pontypool to Neath. Lying on the section between Treharris and Aberdare, the route parallels the still in use TVR Cynon Valley Branch. The route was abandoned before the 1960's, altough the uppermost section of the Cynon valley section remained in use as a coal siding untill the late 1970's. The line was only single track. Cefn-Glas tunnel itself cuts through the southern portion of Craig-yr-Efail between the A470 (the twisty bit just north of the Abercynon roundabout) and the A4059 Cynon Valley road and is about 650m long. The tunnel itself is mostly straight, with just the 50m at the western portal curving slightly northwards. It is therefore not possible to see along the length of the tunnel. Although originally fenced off, the mesh fencing at theis portal has long-since gone, leaving only a couple of concrete posts by the entrance. The first 20-30 m are quite debris strewn ( rubbish, not tunnel roof material) although the tunnel itself is mainly dry. Being a single track tunnel, it has a large number of railway worker refuges dug into the walls, some of which are graffiti'd in the sequence 1) Stop! Turn back now!, 2) You F***ng idiot, go back!, 3) Stop! go no further. All are also emblazoned with a red and white skull motif. This sequence lies about 150m into the tunnel. I firstly assumed these were to scare people off as a joke, but despite the comical nature of the graffiti, this tunnel IS DANGEROUS!!! Parts of the brick lining towards the centre of the tunnel have collapsed. Due to the number of brick linng repair marks, collapse was obviously a problem even when tue tunnel was in use. Although the tunnel is not blocked, more bricks may fall at any time. Since some of the strength of the tunnel roof is within the brick lining, this weakness could cause the tunnel to collapse entirely at some stage. The tunnel lies directly over a former coal mine, and one of the refuges appears to intersect a mine tunnel, but this is collapsed, with old wood in the debris.

The tunnel is lined in part with stone blocks, these belong to the Pennant Sandstone group, mid Carboniferous age that lie stratigraphically above the coal measures. The hard Pennant Sandstone forms the hills between the coalfield valleys. Portions nearer the centre of the tunnel are bare rock, with only the roof arch being bricked. In parts this looks quite clean, but in others, mineralisation has covered the brickwork. Towards the east portal, a large volume of water flows into the tunnel after heavy rain. The floor was also very wet as a the east cutting drains into the tunnel. There are occasional drainage pits in the tunnel floor, obviously drainage was a persistant problem here.

A visit in 2006 revealed that at least the east portal has been fenced off. The photogallery is composed from 4 trips and runs from east to west.