"We should have left this part of the island alone."
— Sir Topham Hatt, Rusty and the Boulder, fifth series
Boulder Quarry was one of the most short-lived industries on the Island of Sodor. It is located off the Ulfstead Branch Line.
Built up in the hills, the quarry was operated by the narrow gauge engines, who collected rock and transported it to the exchange sidings of the North Western Railway. However, when Thumper was digging into a mountain with a boulder perched on top of it, the boulder fell down the mountain and landed on the track, causing chaos and runaways. After this, Sir Topham Hatt decided to close down the quarry, declaring that it was a part of the island that should have been left alone. George, Terence and Butch have also worked here.