A booster engine for steam locomotive

Steam locomotive

A steam locomotive is a railway locomotive that produces its power through a steam engine. These locomotives are fueled by burning some combustible material, usually coal, wood or oil, to produce steam in a boiler, which drives the steam engine...

s is a small two-cylinder steam engine

Engine

An engine or motor is a machine designed to convert energy into useful mechanical motion. Heat engines, including internal combustion engines and external combustion engines burn a fuel to create heat which is then used to create motion...

 back-gear-connected to the trailing truck axle on the locomotive or, if none, the lead truck on the tender. A rocking idler gear permits it to be put into operation by the engineer. It would drive one axle only and could be non-reversible with one idler gear or reversible with two idler gears.

They were used to start a heavy train or maintain low speed under demanding conditions. It could be cut in while moving at speeds under 15 mph (24 km/h). Rated at about 300 hp (224 kW) at speeds of from 10 to 30 miles per hour (16 to 48 km/h). It would automatically cut out at 30 mph (48 km/h). Tractive effort of 10-12,000 lbf (44 to 53 kN) was common.

Tender boosters were equipped with side-rods connecting axles on the lead truck. Such small side-rods restricted speed and thus confined to switching locomotives, often used in transfer service between yards. Such boosters were far rarer than engine boosters.

Reasons for booster use

The booster is intended to address fundamental flaws of the standard steam locomotive. First, most steam locomotives do not provide power to all wheels. The amount of force that can be applied to the rail depends on the weight on driven wheels and the factor of adhesion

Factor of adhesion

In railroad engineering, the factor of adhesion of a locomotive is the weight on the driving wheels divided by the starting tractive effort.A common rule is that for a steam locomotive a good factor of adhesion equals or exceeds 4, but not by too much...

 of the wheels against the track. Unpowered wheels effectively 'waste' weight which could be used for traction. Unpowered wheels are generally needed to provide stability at speed, but at low speed this is not necessary.

Second, the "gearing" of a steam locomotive is constant, since the pistons are linked directly to the wheels via rods and cranks. Since this is fixed, a compromise must be struck between ability to haul at low speed and the ability to run fast without inducing excessive piston speeds (which would cause failure) or the exhaustion of steam. This compromise means that the steam locomotive at low speeds is not able to use all the power the boiler is capable of producing; it simply cannot use steam that quickly, and there is a big gap between the amount of steam the boiler could produce and the amount that can be used. The booster enables that wasted potential to be put to use.

Disadvantages

Boosters were costly to maintain with their flexible steam and exhaust pipes, idler gear etc.

Usage

The booster saw most use in North America

North America

North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

. Railway systems elsewhere often considered the expense and complexity unjustified.

Even in the North American region, booster engines were applied to only a fraction of all locomotives built. Some railroads used boosters extensively while others did not. The New York Central was a fan of the booster and applied it to all of its 4-6-4

4-6-4

Under the Whyte notation for the classification of steam locomotives, 4-6-4 represents the wheel arrangement of four leading wheels on two axles , six powered and coupled driving wheels on three axles, and four trailing wheels on two axles .Other equivalent classifications are:UIC classification:...

 Hudson

NYC Hudson

Hudson was the name given to the 4-6-4 steam locomotive wheel arrangement by the New York Central Railroad which was the first to use locomotives of this type in North America.-History:...

 locomotives. The rival Pennsylvania Railroad

Pennsylvania Railroad

The Pennsylvania Railroad was an American Class I railroad, founded in 1846. Commonly referred to as the "Pennsy", the PRR was headquartered in Philadelphia, Pennsylvania....

, however, used few booster-equipped locomotives.

Canadian Pacific Railway

Canadian Pacific Railway

The Canadian Pacific Railway , formerly also known as CP Rail between 1968 and 1996, is a historic Canadian Class I railway founded in 1881 and now operated by Canadian Pacific Railway Limited, which began operations as legal owner in a corporate restructuring in 2001...

 rostered 3,257 steam locomotives acquired between 1881 and 1949, yet only 55 were equipped with boosters. 17 H1 class 4-6-4s, 2 K1 class 4-8-4

4-8-4

Under the Whyte notation classification of steam locomotives, 4-8-4 represents the wheel arrangement of four leading wheels on two axles , eight powered and coupled driving wheels on four axles, and four trailing wheels on two axles .Other equivalent classifications are:UIC classification: 2D2...

s and all 36 T1 class 2-10-4

2-10-4

Under the Whyte notation for the classification of steam locomotives, a 2-10-4 locomotive has two leading wheels, ten driving wheels , and four trailing wheels...

s.

In Australia, Victorian Railways

Victorian Railways

The Victorian Railways operated railways in the Australian state of Victoria from 1859 to 1983. The first railways in Victoria were private companies, but when these companies failed or defaulted, the Victorian Railways was established to take over their operations...

 equipped all but one of its X class

Victorian Railways X class

The X class was a mainline goods locomotive of the 2-8-2 'Mikado' type that ran on the Victorian Railways between 1929 and 1961. They were the most powerful goods locomotive on the VR until the advent of diesel-electric traction, and operated over the key Bendigo, Wodonga, and Gippsland...

 2-8-2

2-8-2

Under the Whyte notation for the classification of steam locomotives, 2-8-2 represents the wheel arrangement of two leading wheels on one axle , eight powered and coupled driving wheels on four axles, and two trailing wheels on one axle...

 locomotives (built between 1929 and 1947) with a 'Franklin' two cylinder booster engine after the successful trial of the device on a smaller N class

Victorian Railways N class

The N class was a branch line steam locomotive that ran on Victorian Railways from 1925 to 1966. A development of the successful K class 2-8-0, it was the first VR locomotive class designed for possible conversion from 5 ft 3 in broad gauge to 4 ft 8½ in standard gauge.-History:In 1923, in...

 2-8-2

2-8-2

Under the Whyte notation for the classification of steam locomotives, 2-8-2 represents the wheel arrangement of two leading wheels on one axle , eight powered and coupled driving wheels on four axles, and two trailing wheels on one axle...

 in 1927. The South Australian Railways

South Australian Railways

South Australian Railways built and operated railways in South Australia from 1854 to the incorporation of its non-urban railways into the Australian National Railways Commission in 1975, together with the former Commonwealth Railways and the former Tasmanian Government Railways...

 500 class 4-8-2

4-8-2

Under the Whyte notation for the classification of steam locomotives, 4-8-2 represents the wheel arrangement of four leading wheels on two axles , eight powered and coupled driving wheels on four axles, and two trailing wheels on one axle...

 heavy passenger locomotives were rebuilt into 4-8-4

4-8-4

Under the Whyte notation classification of steam locomotives, 4-8-4 represents the wheel arrangement of four leading wheels on two axles , eight powered and coupled driving wheels on four axles, and four trailing wheels on two axles .Other equivalent classifications are:UIC classification: 2D2...

s with the addition of a booster truck from 1929 onwards.

NZR

New Zealand Railways Department

The New Zealand Railways Department, NZR or NZGR and often known as the "Railways", was a government department charged with owning and maintaining New Zealand's railway infrastructure and operating the railway system. The Department was created in 1880 and was reformed in 1981 into the New...

's Kb class

NZR Kb class

The NZR KB class of 1939 was a class of mixed traffic steam locomotives that operated on New Zealand's railway network. They were built by the New Zealand Railways Department after the success of the K class to meet the increasing traffic demands on the Midland Line in the South Island...

of 1939 were built with booster trucks to enable the locomotives to handle the steeper grades of the South Island lines.