By the early 1860s Llan Ffestiniog had largely become a dormitory town supplying workers to the slate industry around Blaenau Ffestiniog. The quarries at Blaenau were connected to the harbour at
Porthmadog by the recently built Festiniog Railway (FR) and as a result were able to significantly increase their output and profitability. However, the FR was struggling to cope ("hopelessly overburdened" according to the standard work on the F&BR) with the volume of traffic from the quarries and some owners – notably
Samuel Holland, later to be a local MP – were looking for other outlets for their product at lower charges. At the same time, several
standard gauge railway companies were looking to extend their lines into the region to tap the demand for slate transport. In particular the
GWR-backed
Corwen and Bala Railway reached
Bala, about 22 miles south of Blaenau in April 1868 and had ambitions to spread further. The
LNWR were also making moves from the north, but that played no clear part in the F&BR story. The quarries around Tanymanod – especially
Craig Ddu – were not rail connected and were on the proposed route of a northward extension of the Corwen and Bala Railway. In October 1866 some quarry owners issued a prospectus announcing the formation of the Festiniog and Blaenau Railway Company to construct a line from Llan Ffestiniog to Blaenau where it would connect with the Ffestiniog Railway. In the short term this "poured water into a blocked drain" as the F&BR led only to the overstretched FR, but the line contained the seeds of at least two ambitions-cum-threats: it took a step towards the south where it might eventually meet the Corwen and Bala's ambitions from the south east and the same step south lent itself to ambitions to connect with the
Cambrian Railways to the southwest. This latter bore at least legal fruit when the
Merionethshire Railway gained parliamentary approval in the
Merionethshire Railway Act 1871 (
34 & 35 Vict. c. lxxii). That line would have continued from Llan Ffestiniog to a junction just north of , but no construction ever happened, "the threat of it had played its part". The prospectus made brief mention of the line being able to use the FR to "command a very large traffic in goods", but saved its most lavish imprecations for the passenger potential, notably for workmen ("very great") and tourists ("immense"). Although initially promoted as, in effect, a branch of the FR, the F&BR was built on a formation that was designed to be easily
converted to standard gauge if and when an extension from the Corwen and Bala Railway approached from the south. Later writers sometimes look at the line's standard gauge successor's sinuous course and conclude it betrays its narrow gauge origins, but the trackbed and bridges were built to accommodate standard gauge trains without alteration and the line's curves' ruling minimum radius was as opposed to the FR's . Even the line's steepest gradient of 1 in 68 was forced upon it by having to avoid obstacles, the original intention was to have a ruling gradient of 1 in 100. The maximum line speed of was imposed by the Board of Trade. The largest feature which had to be rebuilt on conversion was the wooden viaduct between Tanymanod and Blaenau, not because it was too small, but because it was too flimsy. It was rebuilt in stone and stands to this day. Some bridges were erected during the line's later narrow gauge years and on conversion, using metal parapets pre-cast at the Brymbo Foundry. Others of the type also existed on the standard gauge line approaching from Bala. In 2016 at least one such survived between Llan Ffestiniog and Trawsfynydd. There were connections at Board level between these railways and Brymbo. ==Opening==