During the Detailed Examination in May this year, Peter Harris, a tunnels expert from the Institute of Civil Engineers, went underground and examined the tunnel. His report on it has been accepted, and it has been added to the list of structures of historic engineering importance. Peter appears on the video that Graeme Bickerdike made of the detailed examination, available from this website (go to media->video->detailed examination). You can read the details of why it is important by following the link below.
Link to ICE Listing of Rhondda Tunnel. You need to download the complete information which gives a history of the tunnel.