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Added multiple units and hydraulic transmissions
The arrangement of wheels on steam locomotives and small diesel locomotives is normally described using [[Whyte notation]] which specifies the number of powered and unpowered wheels, for example 0-6-0 or 4-6-2. This system does not work well for the more complicated arrangements found in large diesel locomotives or multiple units which may have bogies containing both powered and unpowered wheels and/or using single or multiple traction motorsor hydraulic transmissions. Their wheel arrangement is therefore normally described using the more flexible '''UIC classification system''' which denotes the number of axles (as opposed to wheels) and specifies whether each is powered or unpowered.
==The international standard UIC notation==
*Where the bogie contains several different axle types, '''Brackets''' are used to group together letters and numbers describing the same bogie or truck. Thus (A1A)(A1A) indicates two three-axle bogies, each with the outer two axles driven by separate traction motors and an unpowered central axle.
Although not present on the SVR, the UIC classification system also describes articulated vehicles (where the inner bogie is shared between two vehicles). So, 2'2'2 is used to describe an articulated pair of unpowered vehicles in the [https://en.wikipedia.org/wiki/British_Rail_Class_745 Stadler Class 745units]. Where articulated sets are joined within a unit a '''plus sign''' sign is used, thus Bo'2'2+2'2'2.
===Commonwealth classification===
