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The Shay locomotive was the most widely used geared steam locomotive. They were built to the patents of Ephraim Shay, who can be fairly credited with the popularization of the concept of a geared steam locomotive. Although the design of Ephraim Shay's early locomotives differed from later ones, there is a clear line of development that joins all Shays.

Class C *Sonora* (three driven trucks and articulated tender)

Shay locomotives were built by Lima Locomotive Works of Lima, Ohio, USA, although six Shay Patent locomotives, known as Henderson style Shays, were built by the Michigan Iron Works in Cadillac, Michigan, USA. Geared locomotives built by Willamette and other manufactures are often incorrectly referred to as Shays.

Drive side of the Class B Shay locomotive *Dixiana* at the Roaring Camp and Big Trees Railroad, Felton, California

Shay locomotives had regular fire-tube boilers offset to the left to leave room for a two or three cylinder "motor," mounted vertically on the right with longitudinal driveshafts extending fore and aft from the crankshaft at wheel axle height. These shafts had universal joints and square sliding slip joints to accommodate motion of swivelling two axle trucks. Each axle was driven by a separate bevel gear and used no siderods.

Driving all wheels, even those of the tender, together with small diameter wheels were the strength of these engines, their entire weight developing tractive effort. A high ratio of piston strokes to wheel revolutions allowed them to run at partial slip, where a conventional rod engine would spin its drive wheels and burn rails, losing all traction on molten steel.

Shay locomotives were often known as sidewinders or stemwinders for their side-mounted driveshafts. Most were built for use in the United States, while many found their way to 30 additional countries, territories or provinces.

Although the Shay was the most common geared locomotive, it had a significant flaw that was not recognized or corrected by the manufacturer. Because the drive shaft lies outside the trucks, instead of in the center, truck rotation when following track curvature causes substantial driveline length change, unlike the central drive shafts of Heisler and Climax locomotives. In modern drive shafts, this effect is accommodated by roller splines instead of bronze slip joints (shown between "Sonora's universal joints") that lose their ability to slide under high torque.

Wreck photographs in logging literature show Shay locomotives, before or after uphill curves, where they failed to respond to change in track curvature, thereby running off the track "for no apparent reason." Some texts refer to these locomotives as "rail spreaders" and "flange hounds," both characteristics of trucks that do not steer freely with heavy drive shaft torque.

Classes

2768 Shay locomotives were built by Lima in four classes, from 6 to 160 tons between 1878 and 1945.

Class A: two cylinders, two trucks. Weights between 6 and 24 tons.
Class B: three cylinders, two trucks. Weights between 10 and 80 tons.
Class C: three cylinders, three trucks. Weights between 40 and 160 tons.
Class D: three cylinders, four trucks. Weights of 100 and 150 tons.

Note: Two 15 ton Shays were built with two cylinders and three trucks.

Twenty Class D shays were built. They were no more powerful than Class C, but had greater fuel and water capacity resulting in improved adhesion.

Four Shays were built left handed, all special ordered for the Sr. Octaviano B. Cabrera Co., San Luis de la Paz, Mexico.

Survivors

Only 115 Shays survive today, some a combination of parts of two Shays. The oldest surviving Shay sn-122, built in 1884, is currently displayed in Redding CA, at Turtle Bay Exploration Park. The last Shay, sn-3354 built in 1945, still operates on the Cass Scenic Railroad State Park. One of the largest Shays built, this 150 ton Class C locomotive was in service only five years when it was retired and placed in the Baltimore & Ohio Museum. In 1981 it was removed from static display, in exchange for a smaller Shay and a Porter locomotive, and placed in service on the Cass Scenic Railroad. It has now served in tourist and enthusiast service for a longer period than it did for its original owners. It is still in near-new condition.

The Roaring Camp and Big Trees Narrow Gauge Railroad operates the class B Dixiana and the class C Sonora