Evolution of Wheel Layout
The earliest steam locomotives had two or three axles, one or more of which carried the driving wheels. Richard Tr ingenious arrangement which connected the two driving axles to the driving pistons by means of a series of large cog wheels.
In many cases the inclined cylinders drove one pair of large driving wheels directly and these were sometimes lin rods. As locomotives grew in size, weight and power additional wheels were introduced largely to carry the extra weight of water and coal which was needed for the ever increasing journey length. Locomotive designers needed to get as much weight onto driving wheels as reasonably possible to avoid wheel slipping or spinning, a characteristic of steam engines. Heavy individual axle loads however were most undesirable from the point of view of supporting brides and structures. As in all engineering design, this has always meant that some compromise needs to be made between operational desirability and practical structural considerations.
The introduction of electric and diesel multiple units has allowed the use of many more driving wheels along the length of a train, thus reducing the adhesion, acceleration and braking problem.