Cefn-Glas Tunnel was opened in 1851 to take the extension of the GWR route from Pontypool to Neath. It was closed to traffic in 1964. The west portal is located just off the A470 just north of the Abercynon roundabout. It's a fairly straight tunnel 650m long with just a short curve at the Abercynon end. There's been a few falls in places but it's still passable but if you are going to walk through please remember, some rocks have already fallen - this is a DANGEROUS tunnel.
You may notice that in some of the refuges 'mining' has taken place and in at least one a seam of coal is visible. It's said that during the miners strike locals were digging coal from here. There's also some interesting graffiti in some of the rufuges!