A booster engine for steam locomotives is a small two-cylinder steam engine 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. A booster engine was 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 were confined to switching locomotives, often used in transfer service between yards. Such boosters were far rarer than engine boosters.
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