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 drive shafts 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 swiveling two axle trucks. Each axle was driven by a separate bevel gear and used no side rods.
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 drive shafts. 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 drive line length change, unlike the central drive shafts of Heisler locomotives 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.
http://upload.wikimedia.org/wikipedia/en/b/bc/SonoraUJoints.jpg
TPBM has a log cabin....