Editing Booster engine (section)

{{for|the term in rocketry|Booster (rocketry)}}
[[Image:Franklin-Booster.jpg|thumb|300px|Booster engine with the cover removed to show the mechanism.  The driven axle is on the right; the booster normally hangs behind it.]]
[[Image:Booster-Layout.jpg|thumb|300px|Diagram showing how a booster is installed and connected.]]

A '''locomotive booster''' for [[steam locomotive]]s is a small supplementary two-cylinder [[steam engine]] back-gear-connected to the [[trailing wheel|trailing truck axle]] on the locomotive or one of the trucks on the [[tender (rail)|tender]].  It was invented in 1918 by Howard L. Ingersoll, assistant to the president of the [[New York Central Railroad]].<ref>{{cite magazine |url=https://archive.org/details/sim_railway-locomotives-and-cars_1924-10_98_10/page/590/mode/1up?view=theater |title=Locomotive Boosters |magazine=Railway Mechanical Engineer |volume=98 |issue=10 |date=October 1924|publisher=Simmons-Boardman Publishing Corporation |page=590-591 |via=Internet Archive }}</ref>

A rocking [[idler gear]] permits the booster engine to be put into operation by the [[Train driver|driver (engineer)]]. A geared booster engine drives one axle only and can be non-reversible, with one idler gear, or reversible, with two idler gears. There were variations built by the Franklin company which utilized side rods to transmit [[tractive force]] to all axles of the booster truck. These rod boosters were predominately used on the leading truck of the tender, though there is an example of a [[Lehigh Valley Railroad|Lehigh Valley]] [[4-8-4]] using it as a trailing tender truck.<ref>{{Cite web |last=Van Nort |first=Darryl E |date=28 March 2023 |title=Class T-2a 5210 |url=http://www.llarson.com/steam/schenzinger/images/NA96.jpg |access-date=28 March 2023}}</ref>

A booster engine is used to start a heavy train or maintain low speed under demanding conditions. Rated at about {{convert|300|-|500|hp}} at speeds from {{cvt|10|to(-)|35|mph}}, it can be cut in while moving at speeds under {{cvt|12|-|22|mph}} and is semi-automatically cut out via the engineer notching back the reverse gear or manually through knocking down the control latch up to a speed between {{cvt|21|and(-)|35|mph}}, depending on the model and gearing of the booster. A tractive effort rating of {{convert|10000|-|12,000|lbf|kN}} was common, although ratings of up to around {{convert|15,000|lbf|kN|abbr=on}} were possible.<ref name="BoosterTest1">{{cite magazine |url=https://archive.org/details/sim_railway-locomotives-and-cars_1922-10_96_10/page/562/mode/2up?view=theater |title=Dynamometer Tests of the Locomotive Booster |magazine=Railway Mechanical Engineer |volume=96 |issue=10 |date=October 1922|publisher=Simmons-Boardman Publishing Corporation |page=562-565 |via=Internet Archive }}</ref><ref name="BoosterTest2">{{cite magazine |url=https://archive.org/details/sim_railway-locomotives-and-cars_1944-09_118_9/page/399/mode/1up?view=theater |title=Dynamometer Tests of Type E Booster |magazine=Railway Mechanical Engineer |volume=118 |issue=9 |date=September 1944|publisher=Simmons-Boardman Publishing Corporation |page=399-401, 408 |via=Internet Archive }}</ref>

Tender boosters are equipped with side-rods connecting axles on the lead truck. Such small side-rods restrict speed and are therefore confined mostly to switching locomotives, often used in transfer services between yards. Tender boosters were far less common than engine boosters; the inherent weight of the tenders would decrease as coal and water were consumed during operation, effectively lowering the adhesion of the booster-powered truck.