Complex sentence: The Royal Commission made no comment about small to-be-called "narrow gauge" lines, such as the Ffestiniog Railway, which allowed a future multiplicity of narrow gauges in the UK.
Split into simpler sentences: The Royal Commission made no comment about small lines narrower than standard gauge (to be called "narrow gauge"), such as the Ffestiniog Railway. Thus it permitted a future multiplicity of narrow gauges in the UK.