Purpose
The coupled driving wheels on steam locomotives (often simply called "drivers") were held in a straight line by the locomotive's frame. The flanges of the drivers were spaced a bit closer than the rail gauge, and they could still fit between the rails when tracking through a mild curve. At some degree of curvature, though, the flanges on the center driver would begin to bind in the curve. The farther apart were the front and rear drivers, the less track curvature the locomotive could negotiate. One solution was to make the center driver(s) without flanges on the tires. The other solution was to allow at least one of the axles (often the front driver) to move lateral to the frame, and such designs incorporated various devices to permit this motion.
Read more about this topic: Lateral Motion Device
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