Those bearing can be divided into two main subgroups: those which are mounted on the wheels of the wagons and locomotives; and those which are used for the power aggregates – engines, reducers and mechanisms. All of them have a unique status under EN12090 in the European Union, and respectively in Bulgaria, and they have a predetermined resource, in millions of kilometres. For example, trams – a minimum of 1.5 million kilometres, passenger wagons – 3 million kilometres.