A train wheel is part of a complex assembly called a wheelset, and each component has a specific technical name. The Tread is the slightly conical surface that actually makes contact with the rail; its cone shape is vital for "self-steering" the train around curves. On the inner side of the wheel is the Flange, a raised edge that prevents the train from derailing by keeping the wheels locked between the rails. The central portion that connects to the axle is the Hub (or "boss"). Some wheels are "monobloc" (one single piece of steel), while others use a separate Tyre (a replaceable steel ring around the perimeter) to allow for easier maintenance when the surface wears down. These wheels are press-fitted onto the Axle, a solid steel shaft that ensures both wheels rotate at exactly the same speed. The entire unit sits within a Bogie (or truck) undercarriage, which uses Bearings to reduce friction and allow the heavy assembly to spin smoothly under the weight of the train.