B-B and Bo-Bo are the Association of American Railroads (AAR) and British classifications of wheel arrangement for railway locomotives with four axles in two individual bogies. They are equivalent to the B′B′ and Bo′Bo′ classifications in the UIC system. The arrangement of two, two-axled, bogies is a common wheel arrangement for modern electric and diesel locomotives. Bo-Bo Bo-Bo is the UIC indication of a wheel arrangement for railway vehicles with four axles in two individual bogies, all driven by their own traction motors. It is a common wheel arrangement for modern electric and diesel-electric locomotives, as well as power cars in electric multiple units. Most early electric locomotives shared commonalities with the steam engines of their time. These features included side rods and frame mounted driving axles with leading and trailing axles. The long rigid wheelbase and the leading and trailing axles reduced cornering stability and increased weight. The Bo-Bo configuration allowed for higher cornering speeds due to the smaller rigid wheelbase. Furthermore, it allowed better adhesion because all the wheels were now powered. Due to the absence of frame mounted wheels no leading or trailing axles were necessary to aid cornering, reducing weight and maintenance requirements. Due to the advent of modern motors and electronics more power can be brought to the rail with only a few axles. Modern electric locomotives can deliver up to 6400 kW on only four axles. For very heavy loads, especially in transportation of bulk goods, a single unit with this wheel arrangement tends to have too little adhesive weight to accelerate the train sufficiently fast without wheelslip. Bo-1-Bo Eighteen of the Japanese narrow-gauge Bo-Bo electric were rebuilt in the late 1970s to form the Class ED62. An additional carrying axle was added between the bogies to give a B-1-B (AAR) or Bo′1Bo′ (UIC) arrangement. The intention was to give a lighter axle loading for the Iida Line. Bo-2-Bo Another rare arrangement was the Bo-2-Bo used for two gauge Japanese diesel-electric classes, the ED76 and ED78. These used flexicoil outer bogies which permitted the bogies some lateral movement, as well as swivelling. Bo′Bo′+Bo′Bo′ These are a pair of Bo′Bo′ locomotives semi-permanently coupled as a single unit. They are each constructed with a single cab, giving a cab at each end. This layout includes the Alstom Prima II, one of the most powerful electric locomotives in production (). Versions include the China Railways HXD2 and