Co-Co locomotive
Co-Co is the wheel arrangement for diesel and electric locomotives with two six-wheeled bogies with all axles powered, with a separate traction motor per axle. The equivalent UIC classification (Europe) for this arrangement is Co′Co′, or C-C for AAR (North America). Use 
Co-Cos are most suited to freight work as the extra wheels give them good traction. They are also popular because the greater number of axles results in a lower axle load to the track. History 

The first mainline diesel-electric locomotives were of Bo-Bo arrangement. As they grew in power and weight, from 1937 the EMD E-units used an A1A-A1A layout with six axles to reduce axle load, but only four of them were powered. After WWII, the British LMS ordered two prototype locomotives with some of the first Co-Co arrangements. The first C-C design recorded was a narrow-gauge Hornsby opposed-piston Hornsby-Akroyd-engined locomotive of 1903 for the Chattenden and Upnor Railway. There was a two-speed mechanical transmission with drive shafts to the bogies and the axles on each bogie were linked by coupling rods. Variants

Electric locomotives 

There were initially few electric locomotives with this wheel arrangement, as they are usually lighter than diesel-electrics of similar power and so could manage a similar axle loading with a simpler Bo-Bo arrangement. Some of the few early examples were the French CC 7100 of 1949 and the British Railways EM2 of 1953. As high-speed electric locomotives in the 1980s began to achieve powers in the 6,000 hp range, new Co-Co designs appeared, as more axles were needed to distribute this high power. The BR class 92 was a predominantly freight locomotive of this arrangement for the Channel Tunnel, although the passenger Eurotunnel Class 9 instead use a Bo-Bo-Bo arrangement. This provides the same number of axles for traction, although with shorter bogie wheelbases and so gives a smoother ride. C-C 

In C-C (Commonwealth) or C′C′ (UIC) arrangements, the axles of each bogie are coupled together. This may be for either a diesel-hydraulic transmission with a mechanical drive shaft to the bogie and final drives to each axle. Otherwise a monomotor bogie with a single traction motor. These are used for both electrics and diesel-electrics. Co+Co 

Co+Co is the code for a similar wheel arrangement but with an articulated connection between the bogies. The buffer and drawbar forces are taken between the bogies rather than through the frame. These were mostly popular in South Africa. 1Co-Co1 

The 1Co-Co1 wheel arrangement is an alternative to the Co-Co arrangement which has been used where it was desired to reduce axle load. Each 'Co' bogie has an additional non-powered axle in an integral pony truck to spread the load. As the pony truck is articulated within the bogie, the arrangement is (1′Co)(Co1′) in UIC notation. This rare arrangement was used primarily in Britain with the development of the Bollen bogie; on the Southern Railways' first three prototype mainline diesel-electric designs, 10201–10203, and then on production vehicles in British Rail's Class 40 and "Peaks" (BR classes 44, 45, and 46). 1Co+Co1 

1Co+Co1, like Co+Co, is an articulated variant where the drawbar forces are taken between the bogies rather than through the frame. These were used in South Africa, for lighter loadings on the lightly laid . A number of Japanese electrics from the 1930s, also on Cape gauge, such as the EF10 also used this arrangement. 2Co-Co2 

The New Zealand DF class were built in the mid-1950s by English Electric in Britain, as the first diesels for the   New Zealand railways. They were derived from the earlier English Electric 1Co-Co1 bogie design, but to provide increased flexibility for the long wheelbase bogie they used a four-wheeled bogie with more side play, rather than a pony truck. See also
 Co-Bo, which has two uncoupled bogies

References

 
 Co-Co
CC,Co-Co