A geared steam locomotive is a type of steam locomotive which uses gearing, usually reduction gearing, in the drivetrain, as opposed to the common directly driven design. 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. Unlike conventional steam locomotives, they are not classified by their wheel arrangement. Instead, they are classified by their model and the number of trucks they have.

Explanation and rationale

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.

Types of geared locomotive

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 region, 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 Coalbrookdale 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

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 Climax locomotive

Classes B and C Climax locomotives have 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

The Heisler locomotive has a 'V-twin' style steam engine, one cylinder each side of the boiler, affixed to a centrally located longitudinal driveshaft, again geared to the wheels.

Other types

Besides the three main designs mentioned, there were other designs and clones:

Today

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. These locomotives' particular advantage in cane sugar operations is their 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.

Australia

Canada

Indonesia

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.

New Zealand

Te Awamutu "Climax, makers no. 1317 (under static restoration 2014)

United Kingdom

About 30 Sentinels and a few Aveling & Porters have been preserved. A few examples are shown below: The Aveling & Porter 2-2-0WT Blue Circle has changed ownership many times. Last known location - Rushden, Higham and Wellingborough Railway.

United States

No geared steam locomotives remain in commercial use in America. However, several are in operation on tourist lines.

Taiwan

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.