In this way smoothness of working is ensured, the cog-wheel being constantly in action with the rack.
Up the steep slope of Mount Washington runs a cog railway.
In rack railways a cog-wheel on the engine engages in a toothed rack which forms part of the permanent way.
The Puerto Cabello and Valencia line (34 m.) is another British undertaking and carries a good traffic. A part of this line is built with a central cog-rail.
Racks of this type usually become impracticable for gradients steeper than 1 in 4, partly because of the excessive weight of the engine required and partly because of the tendency of the cog-wheel to mount the rack.