Which mountain railway is located on Sodor?

The mountain railway located on Sodor is the Culdee Fell Railway.

In the fictional geography of The Railway Series and Thomas & Friends, the Culdee Fell Railway is a narrow-gauge rack-and-pinion line climbing Culdee Fell, Sodor’s highest mountain. It links with the standard-gauge railway at Kirk Machan and serves the summit and surrounding mountain routes.

Its design is based on real mountain railways, especially Wales’s Snowdon Mountain Railway. The rack-and-pinion system gives trains extra grip on steep gradients, where ordinary adhesion railways would be more difficult to operate safely. The railway and its engines feature prominently in the book Mountain Engines.

The other options are also Sodor railways, which makes this question easy to confuse. The Ffarquhar Branch is a standard-gauge branch line, the Arlesdale Railway is a 15-inch-gauge miniature line, and the Skarloey Railway is another narrow-gauge railway—but the Culdee Fell line is specifically the mountain railway.