Since the beginning of the railroad, locomotives have been classified to identify them clearly. Classification by number and placement of axles became the standard. It permitted identifying the type of locomotive directly from the wheels , counted from front to back: the number of leading idlers (small wheels) + the number of driving axles (large wheels) + the number of rear idlers (small wheels). The Americans and British count wheels, the French count axles and other countries use a number of systems with letters for the driving axles.
