Co-Co is a code for a locomotive

Locomotive

A locomotive is a railway vehicle that provides the motive power for a train. The word originates from the Latin loco – "from a place", ablative of locus, "place" + Medieval Latin motivus, "causing motion", and is a shortened form of the term locomotive engine, first used in the early 19th...

 wheel arrangement with two six-wheeled bogie

Bogie

A bogie is a wheeled wagon or trolley. In mechanics terms, a bogie is a chassis or framework carrying wheels, attached to a vehicle. It can be fixed in place, as on a cargo truck, mounted on a swivel, as on a railway carriage/car or locomotive, or sprung as in the suspension of a caterpillar...

s with all axles powered, with a separate motor per axle. Co+Co

Co+Co

Co+Co is a code for a locomotive wheel arrangement with two six-wheeled bogies with an articulated connection between them and with all axles powered, with a separate motor per axle. Co-Co is the code for a similar wheel arrangement but without the articulated connection between the bogies....

 is the code for a similar wheel arrangement but with an articulated connection between the bogies. The equivalent UIC classification

UIC classification

The UIC classification of locomotive axle arrangements describes the wheel arrangement of locomotives, multiple units and trams. It is set out in the International Union of Railways "Leaflet 650 - Standard designation of axle arrangement on locomotives and multiple-unit sets". It is used in much...

 for this arrangement is Co′Co′. Co-Cos are most suited to freight work as the extra wheels give them good adhesion

Adhesion

Adhesion is any attraction process between dissimilar molecular species that can potentially bring them in close contact. By contrast, cohesion takes place between similar molecules....

 (traction

Traction

- Engineering :*Forces:** Traction , adhesive friction or force in the context of vehicle** Traction vector, in mechanics, the force per unit area on a surface, including normal and shear components...

 or friction

Friction

Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...

 seems the proper word). They are also popular because the greater number of axles results in a lower axle load

Axle load

The axle load of a wheeled vehicle is the total weight felt by the roadway for all wheels connected to a given axle. Viewed another way, it is the fraction of total vehicle weight resting on a given axle...

 to the track.

Notable examples include the British Rail Class 47

British Rail Class 47

The British Rail Class 47, is a class of British railway diesel-electric locomotive that was developed in the 1960s by Brush Traction. A total of 512 Class 47s were built at Crewe Works and Brush's Falcon Works, Loughborough between 1962 and 1968, which made them the most numerous class of British...

, the Soviet M62 locomotive and the EMD Series 66

EMD Series 66

The Electro-Motive Diesel Class 66 is a series of Co-Co six axle diesel locomotives built by EMD for the European heavy freight market....

, mainstay of many current European heavy rail haulage fleets, over 500 having been built to date. The very strong IORE

IORE

Iore, often stylized IORE, is a class of 26 electric locomotives built by Adtranz and its successor Bombardier Transportation for the Swedish mining company LKAB's railway division Malmtrafik. The class is a variation of Adtranz's Octeon modular product platform, thus related to Bombardier's later...

locomotive has this also, but to allow higher locomotive weight, 30 tonnes per axle.