In Anthony R. Kingdom’s 1991 book, “The Yelverton to Princetown Railway” there is reference to a proposed branch from the GWR Princetown line near King’s Tor to Merrivale Quarry. “In 1908 plans were drawn up by Prigg & Lindon (Engineers) for this railway. It was to be 1 mile, 5 furlongs and 7.25 chains long (about 1.7 miles). It was to be known as the “Merrivale Light Railway” a standard gauge line and its departure point was just past the Swell Tor sidings on the main Yelverton – Princetown line”.
This line never happened – the proposals were wildly impractical (four embankments, two bridges and a cutting was required for the idea) and it’s backers ended up in court facing bankruptcy by 1937 (see here).
You can follow the route here.