Act of Parliament 1921
From Festipedia, hosted by the FR Heritage Group
When there was talk, post WW1, of merging the Railway companies into massive operators, the Festiniog was originally included in the section that would have become part of the GWR
However, at a late point a motion was made, in August 1921, by Lord Clwyd, to have it removed, and this was agreed to, by the Earl of Lytton
The relevant ref is here