A lateral motion device is a mechanism used in some railroad locomotives which permits the axles to move sideways relative to the frame.
[1] The other solution was to allow at least one of the axles (often the front driver) to move laterally relative to the frame, and such designs incorporated various devices to permit this motion.
Typically the bearing boxes were designed to allow the axle to slide some either way, and the wheel cranks and the coupling rods were modified to accommodate this extra range of motion.
[4] Their GS-4 class 4-8-4 also did and employed springs to control the lateral motion, thereby assisting the bogie in easing the engine into curves.
[5] On the Hungarian State Railways class 424 4-8-0s, the rear axle was given 50 mm (2.0 in) of lateral motion "to ease the running on sharp curves.