Lateral motion device
Lateral motion devices permit the axles in some railroad locomotive
A locomotive is a railway vehicle that provides the motive power for a train. The word originates from the Latin loco – "from a place", ablative of locus, "place" + Medieval Latin motivus, "causing motion", and is a shortened form of the term locomotive engine, first used in the early 19th...

s to move sideways relative to the frame.


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
Degree of curvature
Degree of curve or degree of curvature is a measure of curvature of a circular arc used in civil engineering for its easy use in layout surveying....

, 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.


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
Under the Whyte notation for the classification of steam locomotives, 2-6-0 represents the wheel arrangement of two leading wheels on one axle, usually in a leading truck, six powered and coupled driving wheels on three axles, and no trailing wheels. This arrangement is commonly called a Mogul...

. The frontmost axle did not carry a conventional pony truck
Pony truck
A pony truck, in railway terminology, is a leading truck with only two wheels.Its invention is generally credited to Levi Bissell, who devised one in 1857 and patented it the following year. Hence the term Bissel bogie or axle is used in continental Europe...

. Instead this axle along with the front driven axle together carried a bogie
A bogie is a wheeled wagon or trolley. In mechanics terms, a bogie is a chassis or framework carrying wheels, attached to a vehicle. It can be fixed in place, as on a cargo truck, mounted on a swivel, as on a railway carriage/car or locomotive, or sprung as in the suspension of a caterpillar...

. 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
A 4-14-4, in the Whyte notation for the classification of steam locomotives by wheel arrangement, is a locomotive with four leading wheels, fourteen coupled driving wheels in a rigid frame, and four trailing wheels....

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.
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