Officially the line's gauge was , however, a Board of Trade inspection in 1868 recorded it as , as did the locomotives' manufacturer's catalogue, repeated in the magazine "Engineer". A survey of the line conducted by Vignes in 1878 gave the gauge as . In practice the tolerances were sufficiently wide to allow Festiniog Railway locomotives and rolling stock to use the line, but there is no record of F&BR stock venturing onto FR metals other than transit to when their two locomotives made rare visits to Wolverhampton for heavy repair. They are unlikely to have made these trips along the FR in steam and may even have travelled on flat wagons.