Boulder is a giant boulder who lives high in the mountains of Sodor.
His home was reached by the Skarloey Railway, who then preceded to build a quarry on the cliff-side below it. Rusty was suspicious of it, and thought that it may have been watching him. Later, the drilling into the mountain by Thumper caused vibrations that shook Boulder loose from his perch and sent him onto the tracks, creating a path of destruction along the Skarloey Railway, finally terminating by destroying the standard gauge sheds. After this, Sir Topham Hatt closed the quarry and had Boulder moved to a new mountain, close by the yards. It still stands there to this day.
Boulder is perfectly round, and has moss clinging to him in some places. A mysterious face sometimes appears on him. His persona is described as proud and silent.
- Boulder's face is based on Gordon's upset face mask.