1Co-Co1
A wheel arrangement for diesel-electric locomotives signifying that a locomotive has eight axles on two bogies, six axles are powered by electric motors, with one outer axle on each bogie unpowered.
1Co-Co1: six-powered diesel-electric locomotive wheel plan
A 1Co-Co1 locomotive has eight axles arranged on two swiveling bogies (trucks), with the center four axles of each bogie powered by electric traction motors and the outer axles unpowered. This arrangement yields six powered axles total, hence the notation: the outer 1s represent the unpowered axles, while Co-Co denotes three consecutive powered axles on each bogie. The configuration emerged in mid-20th century European railway engineering, particularly for heavy freight and mainline services where tractive effort and adhesion had to be balanced against rail damage and maintenance costs.
The unpowered outer axles, called idler or carrying axles, serve load distribution and guidance functions. They reduce unsprung mass on the powered axles and improve wheel-to-rail contact geometry, particularly on curves and worn track. This prevents the characteristic hunting oscillation that occurs with fully powered bogies at speed. The 1Co-Co1 layout is therefore a mechanical compromise between the all-powered Bo-Bo (four axles, all driven) and heavier arrangements like Co-Co (six powered axles) or 2-Co-Co-2 (eight powered axles with leading/trailing carrying axles).
Traction and adhesion trade-offs
With six of eight axles powered, a 1Co-Co1 can develop substantial tractive effort while keeping adhesion factors within acceptable limits. On dry rail, adhesion limits pull; on wet or contaminated rail, the powered axles may slip before reaching their maximum torque output. The unpowered outer axles do not slip, and their load helps anchor the bogie during hard acceleration. This design proved popular for locomotives in the 50 to 100 tonne class operating on regional and branch line networks where rail conditions were variable and maintenance budgets were tight.
The 1Co-Co1 is less common than Bo-Bo in North America but was widely adopted by European railways, particularly Swiss, German, and Scandinavian operators. Its appearance declined after the 1980s as more homogeneous Bo-Bo, Co-Co, and articulated designs came to dominate new-build orders. However, preserved and heritage railways still operate 1Co-Co1 locomotives from the 1950s and 1960s, and the wheel plan remains current in certain regional freight applications where low axle loads are a requirement.