The long rigid wheelbase of the conventional steam locomotive becomes a problem on tightly curved rail lines, or when the size of the locomotive increases. While leading and trailing wheels are able to move from side to side to accomodate curvature, the movement of driving wheels is restricted. The problem can be alleviated a little by using lateral motion devices on some or all of the driving axles, permitting a small amount of sideplay, and by having blind (flangeless) wheels on some of the driving axles.
These can only be taken so far, however. Some form of articulation was the obvious solution; providing some means or other by which multiple sets of driving wheels could move independently, generally by hinging the locomotive frame in some manner.
Many kinds of articulated locomotives were devised. Fairlies, Mallets, Garratts, and Kitson-Meyers were all designs of articulated locomotive. Geared locomotives like Shays, Climaxes and Heislers were articulated locomotives too.