Bo-Bo

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.

Bo′Bo′ (UIC) or Bo-Bo (Commonwealth) is the indication of a wheel arrangement for railway vehicles with four axles in two individual bogies, all driven by their own traction motors.

The long rigid wheelbase and the leading and trailing axles reduced cornering stability and increased weight.

Due to the absence of frame mounted wheels no leading or trailing axles were necessary to aid cornering, reducing weight and maintenance requirements.

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.

This layout includes the Alstom Prima II, one of the most powerful electric locomotives in production (9 MW (12,000 hp)).

[3][4] In some rare examples, such as the SNCF Class BB 71000 and the narrow-gauge ÖBB 2095 [de], the bogie axles have been linked by coupling rods.

[5] A common arrangement is for each power car to have two independent engines and transmissions, each driving a single axle of each bogie.