Diesel notation
North American diesels are often described using AAR notation. B-B locomotives have two powered axles in each of two trucks; C-C locomotives have three powered axles in each truck.
Why six axles?
Six powered axles spread weight and provide additional traction motors, making C-C arrangements common on heavy road-freight locomotives.
Steam notation
Steam locomotives commonly use Whyte notation. A 4-8-4 has four leading wheels, eight drivers and four trailing wheels; Big Boy's 4-8-8-4 notation shows its articulated driving arrangement.
Identification clue
Counting axles, noting truck shape and comparing body length are useful first steps when identifying an unfamiliar locomotive.
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