Overshadowed by its bigger and more celebrated brother called Rhondda, the 164-yard Gelli Tunnel formed part of a line built to connect the collieries of the Rhondda valley with the Swansea Bay ports. As it transpired, the considerable efforts of its engineers never reaped great financial dividends due to the tortuous gradients involved.
Heading eastwards from Cymmer, trains clung to the northern side of the Afan gorge, disappearing into Gelli Tunnel on a curve.