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Co-Co is the wheel arrangement for a diesel locomotive with two six-wheeled bogies with all axles powered, with a separate motor per axle. The equivalent UIC classification (Europe) for this arrangement is Co′Co′ or C-C for AAR (USA).
Co+Co is the code for a similar wheel arrangement but with an articulated connection between the bogies.
The 1Co+Co1 (and 1Co-Co1) wheel arrangement is a development of the Co-Co arrangement and is used where it is necessary to reduce axle load. Each "Co" bogie has an additional non-powered axle in an integral pony truck to spread the load.
Notable examples include the British Rail Class 47, the Soviet M62 locomotive and the EMD Series 66, mainstay of many current European heavy rail haulage fleets, over 500 having been built to date. The very strong IORE locomotive has this also, but to allow higher locomotive weight, 30 tonnes per axle.