Spider (locomotive)

The term "Spider" as applied to a locomotive is a colloquial and generally unofficial descriptor, often used to refer to steam locomotives with a driving wheel arrangement that features a high number of wheels, particularly when those wheels are relatively small in diameter and arranged closely together. This wheel arrangement often gives the appearance of many legs, thus the association with a spider.

The term is not a formal classification recognised in locomotive engineering or historical records. It is more accurately considered slang, common among railfans and enthusiasts rather than professional railway personnel. Its application is subjective and dependent on the observer's perception.

Locomotives described as "Spider" types are frequently associated with shunting (switching) or industrial applications where high tractive effort at low speeds is prioritised over high-speed running. A high number of coupled driving wheels increases adhesion, allowing the locomotive to pull heavy loads.

The specific wheel arrangement most likely to be referred to as a "Spider" would typically be a 0-8-0 or 0-10-0 (using Whyte notation), although other arrangements with numerous small driving wheels could also be included under this informal label. The designation may also occasionally be applied, albeit less frequently, to tank locomotives with similar wheel arrangements.

It is important to note that the term "Spider" does not denote a specific class, manufacturer, or official type of locomotive.