The eastern spur of the triangle permitted through running from South Wales to mid-Wales and also to Hereford.

The northern side of the triangle followed the course of the 1816 Hay Railway, a tram-road worked by horses connecting the town of Hay with the Brecknock and Abergavenny Canal at Brecon.

At the western end lay a tunnel which required widening and deepening for use by standard gauge trains.

Talyllyn Junction is often quoted as a defining feature of the Great Western Railway in Wales, namely its inheritance of junctions in unlikely and inconvenient locations.

All the railways at Talyllyn Junction were closed to passengers in 1962.