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 the lead truck on the tender. A rocking idler gear permits it to be put into operation by the driver (engineer). It drives one axle only and can be non-reversible, with one idler gear, or reversible, with two idler gears. A booster engine is used to start a heavy train or maintain low speed under demanding conditions. Rated at about at speeds of from , it can be cut in while moving at speeds under and is semi-automatically cut out via the engineer notching back the reverse gear between , depending on the model and design of the booster. A tractive effort of is common, although ratings of up to around were possible. Tender boosters are equipped with side-rods connecting axles on the lead truck. Such small side-rods restrict speed and are therefore confined to switching locomotives, often used in transfer services between yards. Tender boosters were far more rare than engine boosters. Reasons for booster use The booster is intended to make up for fundamental flaws in the design 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 the driven wheels and the factor of adhesion of the wheels against the track. Unpowered wheels are generally needed to provide stability at speed, but at low speed they are not required, so they effectively 'waste' weight which could be used for traction. Second, the "gearing" of a steam locomotive is constant, because the pistons are linked directly to the wheels via rods and cranks. Therefore, 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. That compromise means that, at low speeds, a steam locomotive is not able to use all the power the boiler is capable of producing; it simply cannot use steam that quickly, so there is a big difference between the amount of steam the boiler can produce and the amount that can be used. The booster engine enables that wasted potential to be put to use. Disadvantages Boosters are costly to maintain, with their flexible steam and exhaust pipes,