This gearing is part of the machinery within the locomotive and should not be confused with the pinion that propels a rack locomotive along the rack between the rails. The geared steam locomotives that have been built have been for conventional track, relying on the adhesion between wheels and rail.
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 (steam locomotives are almost universally double-acting, unlike the more familiar internal combustion engine).
The maximum rotational speed is fairly fixed for a given engine technology. Given the lack of any variable-ratio transmission between the piston engine and the wheels, the designer is forced to 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 available starting torque and thus tractive effort will be too small, and the locomotive will not be able to start a train.
Many industrial applications require a low speed locomotive with ample starting tractive effort. These industries range from mining and quarry operations to forestry and logging operations. Steeply graded lines, especially when the track is cheaply built and not suited to high speeds, will also favour the usage of a locomotive with a high tractive effort. Although the trade-off of speed versus torque can be adjusted in favour 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 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 a geared locomotive. Most were and are still single speed, but some did employ a variable-ratio gearbox and multiple ratios.
The vast majority of geared locomotives in the world were built to one of three distinct 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-developed types.
These were not the first locomotives to use geared transmission. Richard Trevithick's Coalbookdale Locomotive used a large gear instead of side rods to link the crankshaft to the driving axles, with a net 1:1 gear ratio. The early Grasshopper (1832), Crab (1837) and Mud Digger (1842) locomotives built for the Baltimore and Ohio Railroad used gear ratios on the order of 2:1 so that each turn of the crankshaft caused about two turns of the driving axles. This allowed use of relatively small driving wheels without sacrificing speed.
The Shay locomotive features 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 (see also Ephraim Shay, inventor).
The Class B Climax locomotive has 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.
There was also an earlier Class A Climax with a vertically mounted marine-type steam engine, working through a similar drive-line, via a two-speed gearbox.
The Heisler locomotive has 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.
Besides the three main designs mentioned, there were other designs and clones:
With the decline of the commercial use of steam traction, the commercial use of geared locomotives has similarly reduced.
Some geared steam locomotives are still at work in the sugar plantations of Indonesia, and no doubt elsewhere too, but in most countries they may now be seen only on tourist lines, preservation sites and museums. The particular advantage in cane sugar operations is the ability to use the dried solid residue of pressing the cane (see bagasse) as a fuel of trivial cost, providing that low cost technical labor is available to maintain the locomotives.
The British Columbia Forest Discovery Centre in Duncan, British Columbia. Shays - Lima shop numbers 2475 (display) and 3147 (out of service pending boiler work). Climax shop numbers 1057 (display) and 1359 (operational).
Wide variety of types still in use at sugar mills. Most are long wheelbase 0-10-0 locomotives that use an articulation technique incorporating a geared drive to the outer-most axles, the inner pair being direct-drive.
Te Awamutu "Climax, makers no. 1317 (under static restoration 2014)
About 30 Sentinels and a few Aveling & Porters have been preserved. A few examples are shown below:
No geared steam locomotives remain in commercial use in America. However, several are in operation on tourist lines.
The Cass Scenic Railroad State Park in West Virginia uses only geared locomotives, and features the largest remaining and last Shay locomotive ever built, the 162-ton former Western Maryland Railway #6. The railroad also owns a Climax locomotive and a Heisler, enabling all three types to be seen.
The White Mountain Central Railroad at Clark's Trading Post in Lincoln, New Hampshire, operates Climax #6, built in 1920 (builder number 1603), on its tourist line. Clark's also has the last remaining Shay in New England. The Shay is non-operational but in storage. Clark's also has a 32-ton Heisler locomotive that is awaiting staybolt replacement.
The Southeastern Railway Museum in Duluth, Georgia, has a two-truck Heisler (#9) lovingly restored on static display inside the main exhibit hall. The Heisler is painted for the Campbell Limestone Co and is in excellent condition.
At the Mount Rainier Scenic Railroad in Washington state, a 99-ton West Fork Logging Heisler #91 is operational, a 70-ton Hillcrest Lumber Climax #10 is operational, a 95-ton Pickering Lumber Shay #11 is awaiting boiler work, and a 75-ton Rayonier Company Willamette #2 is operational and is the only Willamette operating.
The Yosemite Mountain Sugar Pine Railroad in Sugar Pine, California.
The Alishan Forest Railway in Taiwan operated 22 Shay locomotives in the past, with the oldest dating to 1910. Sixteen of the original 22 have been preserved, with 3 in operational condition and 1 preserved on the Puffing Billy Railway.