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.