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. In AAR notation, the notation is the same for both variants and cannot distinguish them. However hydraulic transmissions are almost unknown in the US, the Krauss-Maffei ML 4000 and the ALCO DH643 being the only C′C′ examples.