A Bo+Bo wheel arrangement is an electric locomotive, with two four-wheeled chassis or bogies with an articulated connection between them, with the drawbar forces taken through these bogies, and with all axles powered by individual traction motors.[1]
This is in contrast to the more common Bo-Bo arrangement where the two bogies are mounted beneath a shared frame, and the forces are taken through that frame.
The type was important for early electric locomotives such as the first in 1899[2] or the Italian E.430 of 1901. These had two separate chassis and half-cabs, linked by a pin connector. A canvas dodger joined the gap between the cabs.
Later designs resembled the Bo-Bo layout, where a single large boxcab spanned the two bogies and was pivoted upon them. As these pivots did not carry the tractive force, they could be of simple design.[3]
Typical mid-century Bo+Bo locomotives include the South African Class 1E and Class 2E or the Japanese National Railways . Railways using Cape gauge such as the South African or Japanese narrow gauge systems with long locomotives found Bo+Bo advantageous over Bo-Bo as there was less overhang of the couplers and so less sideways misalignment on tight curves.