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B-B and Bo-Bo are the 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 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.

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