The Mount Washington Cog Railway, also known as the Cog, is the world's first mountain-climbing cog railway (rack-and-pinion railway). The railway is still in operation, climbing Mount Washington in New Hampshire, United States. It uses a Marsh rack system and both steam and biodiesel-powered locomotives to carry tourists to the top of the mountain. Its track is built to a gauge, which is technically a narrow gauge, as it is less than a .