R&SBR
The merchants of Swansea had wanted to tap into the coal fields of the Rhondda Fawr for some time, but proposals brought to them until that point had all required massive construction in the Afan Valley, and steep gradients into Treherbert restricting train loads.
Eventually a proposal was made to cut a tunnel from Blaengwynfi to Treherbert, which would allow easier access to Rhondda Fawr.
Read more about this topic: Rhondda And Swansea Bay Railway