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.
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.
Read more about Booster Engine: Reasons For Booster Use, Disadvantages
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“The machine unmakes the man. Now that the machine is perfect, the engineer is nobody. Every new step in improving the engine restricts one more act of the engineer,—unteaches him.”
—Ralph Waldo Emerson (1803–1882)