The basic concept of railway vehicle wheels has remained unchanged from the earliest days of railway history. The assembly of two wheels and an axle is commonly referred to as a wheelset. The wheels are solid forged steel or cast steel. They can be differentiated into two main categories: monobloc wheels and wheels with attached tires. On steam locomotives, each of them is equipped with a crank connected to the coupling rod, as well as a counterweight (on the opposite side) for balancing bringing the center of gravity back to the axle.