The steam locomotive, as commonly employed, has its pistons directly attached to cranks on the driving wheels; thus, there is no gearing, one revolution of the driving wheels is equivalent to one revolution of the crank and thus two power strokes per piston (the steam locomotive is double acting, unlike the more familiar internal combustion engine).
The maximum rotational speed (measured in revolutions per minute or rpm) is fairly fixed for a given engine technology. Given the lack of any indirect transmission between the piston engine and the wheels in the locomotive, the designer is forced to choose one compromise between desired torque and desired maximum speed; the radius of the driving wheels determines this. The radius of the crank affixed to the wheel is of course less than this; its radius determines the length of the piston stroke. This cannot be too large, for the locomotive will be unable to generate enough steam to supply those large cylinders at speed; it cannot be too small, or the starting torque and thus tractive effort will be too small, and the locomotive will not be able to start a train.
Many applications required a low speed locomotive with ample starting tractive effort; industrial use, mines and quarries and logging operations, steeply graded lines and the like, especially when the track is cheaply built and not suited to high speeds anyway. Unfortunately, although the tradeoff of speed versus torque can be adjusted in favor of torque and tractive effort by reducing the size of the driving wheels, there is a practical limit below which this cannot be done without making the piston stroke too short on a directly driven locomotive.
The solution, of course, is to separate the crank from the wheels, firstly allowing for a reasonable piston stroke and crank radius without requiring larger than desired driving wheels, and secondly allowing for reduction in rotational speed via gearing. Such a locomotive is of course a geared locomotive. Most were and are still single speed, but some did employ a shift-able gearbox and multiple ratios.
Notable geared steam locomotives included:
- The Shay locomotive, featuring an offset boiler with a multiple-cylinder engine affixed to it on the opposite side, driving a longitudinal shaft geared to the axles via bevel gears.
- The Climax locomotive, with two inclined cylinders driving a transverse crankshaft, geared to a longitudinal driveshaft placed centrally on the locomotive and driving the powered trucks via internal gearing.
- The Heisler locomotive, with a 'Vee-Twin' style steam engine, one cylinder each side of the boiler, affixed to a centrally located longitudinal driveshaft, again geared to the wheels.
- The Willamette locomotive was a clone of the Shay locomotive produced after key patents expired. West coast logging customers were clamoring for improvements in detail design and the application of more modern locomotive technology to the geared locomotive; Lima (manufacturers of the Shay) were dragging their heels. The Willamette was the response to that.
The vast majority of geared locomotives in the world were built to one of these three designs, whether licensed and official, or clones built after the expiration of key patents. Of the types, the Shay locomotive was the most numerous and best known. The overwhelming majority operated on the North American continent, but with a number in use in various parts of South America and a fair number in Australia and New Zealand, including home-grown types. Geared locomotives never really took hold in Europe or places in the railway world where the European influence held sway.
Nowadays, none to my knowledge are in commercial use; however, one can still catch a geared locomotive in operation on numerous tourist lines. The Cass Scenic Railroad in West Virginia is probably the best example worldwide, being entirely hauled by geared locomotives, and featuring the largest Shay locomotive in existence, former Western Maryland #6, a 162-ton monster that is effectively still brand new, having seen only four years of operation before retirement. The railroad also owns a Climax locomotive and a Heisler locomotive, enabling all three types to be seen. Also a good place to see geared locomotives is the Roaring Camp and Big Trees Narrow Gauge Railroad in Felton, CA which has several operational Shays; the Georgetown Loop Railroad in Colorado also has operating Shay locomotives.
Facts checked on ShayLocomotives.com and at the Geared Steam Locomotive Works at http://www.trainweb.org/gearedsteam/