I can think of two. There was a sawmill on the Cowbridge Railway, at Ystradowen. In addition, it also had a narrow gauge tramway. The business was owned by a Mr. Roberts, who also loaded timber at St.Athan Road station, and Beaupre House, on the Aberthaw railway. A lot of the London Extension (GCR) sleepers came from the Radnorshire area forests.
Not to mention the Scottish traffic....
Regards,
Ian