Editing Booster engine (section)

== Reasons for use ==
[[File:Reading 2102 Type E Booster.jpg|thumb|300px|Franklin Type-E locomotive booster affixed to [[Reading 2102|Reading T-1 no. 2102]].]]
The booster was intended to make up for fundamental flaws in the design of the standard steam locomotive.  To start off, 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. Therefore, the application of a booster engine to the previously unpowered axle meant that overall starting tractive effort was increased with zero penalty to the adhesion levels of the main engine.

Additionally, the "gearing" of a steam locomotive is fixed, because the pistons are linked directly to the wheels via rods and cranks. Therefore, a compromise must be struck between ability to exert high tractive effort 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 substantial difference between the amount of steam the boiler can produce and the amount that can be used. The booster engine enabled that wasted potential to be put to use.

The increased starting tractive effort provided by the booster meant that, in some instances, railroads were able to reduce the number of, or eliminate the use of additional helper locomotives on heavier trains. This resulted in lower operating and maintenance costs, higher locomotive availability and productivity (ton-miles), and ultimately, greater profitability.