Variations
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. Spring centering might also be provided to keep the wheels on such axles from hunting side to side.
One example was the Italian State Railways class 640 2-6-0. The frontmost axle did not carry a conventional pony truck. Instead this axle along with the front driven axle together carried a bogie. This driving axle had a lateral play of about 20 mm, and spherical bearings were used on the cranks and coupling rods to this axle. (The engine used inside cylinders driving the middle set of drivers.) The first two axles worked together to guide the locomotive, similar to a conventional leading bogie. The Dovregrubben class 2-8-4 on the Norwegian State Railways also used this "Zara" truck.
The Southern Pacific class 5000 4-10-2 employed a conventional leading bogie but used lateral motion devices on the leading driven axle to reduce the rigid wheelbase. 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. On the Hungarian State Railways class 424 4-8-0s, the rear axle was given 50 mm of lateral motion "to ease the running on sharp curves."
The experimental AA20 4-14-4 locomotive was the only one ever to have seven coupled driving axles. It used lateral motion devices (on the first and seventh axles) as well as blind drivers (on the third, fourth and fifth axles) but these measures were not enough to allow it to negotiate curves without damaging the track, derailing, or both.
Read more about this topic: Lateral Motion Device
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