There are
many variations of wheel arrangements for diesel and electric locomotives just as there were with steam locos, and for the same reasons: to facilitate different power outputs and distribute the weight of an engine. These arrangements have been allocated codes to make them easily identifiable.
Co-Co refers to a locomotive with a
single bogie attached to each end, each bogie has 6
wheels attached to it (3 on the left side and 3 on the right). This means that the locomotive has 12 wheels in total. On a Co-Co locomotive, each wheel has its own traction motor resulting in all of the wheels being powered.
Locomotives with a Co-Co wheel arrangement are very good at hauling thanks to their all-wheel drive and often have a favourable route availability due to the weight of the locomotive being distributed through 12 wheels. The latter is the reason why more diesel locomotives have Co-Co wheel arrangements than electrics as these tend to be much lighter, therefore, negating the need to distribute their weight.
There have been a few classes of locomotive that due to their immense weight have needed their weight distributing beyond the capabilities of a Co-Co bogie. This was done by adding an additional two-wheeled, unpowered bogie to each end of the locomotive. This new configuration was given the code 1 Co-Co 1. Locomotives that adopted this configuration include the British Class 40 and Peak classes 44, 45 and 46.