A roundhouse is a building used by railroads for servicing locomotives. Roundhouses are large, circular or semicircular structures that were traditionally located surrounding or adjacent to turntables. The defining feature of the traditional roundhouse was the turntable, which facilitates access when the building is used for repair facilities or for storage of steam locomotives.
Early steam locomotives normally travelled forwards only; although reverse operations capabilities were soon built into locomotive mechanisms, the controls were normally optimized for forward travel, and the locomotives often could not operate as well in reverse. Some passenger cars, such as observation cars, were also designed as late as the 1960s for operations in a particular direction. A turntable allowed a locomotive or other rolling stock to be turned around for the return journey.
Most modern diesel and electric locomotives can run equally well in either direction, and many are push-pull trains with control cabs at each end. In addition, railroads often use multiple locomotives to pull trains, and even with locomotives that have distinct front and rear ends, the engines at opposing ends of a locomotive "consist" (a group of locomotives coupled together and controlled as a single unit) can be aligned so they face opposite directions. With such a setup, trains needing to reverse direction can use a technique known as a "run around," in which the engines are uncoupled from the train, pull around it on an adjacent track or siding, and reattach at the other end. The engineer changes operating ends from the original locomotive to the one on the opposite end of the locomotive consist.
Railroad terminals also use features such as balloon loops and wyes (Commonwealth: triangle) to reverse the orientation of railroad equipment. Because of the advent of these practices, modern roundhouses are frequently not round and are simply large buildings used for servicing locomotives. Like much other railroad terminology, however, the structure has retained its traditional name. The alternative term engine-house encompasses both semi-circular and rectangular structures and broadly describes all buildings intended for storage and servicing of locomotives. Shops or workshops are buildings containing hoists and heavy machinery capable of major repairs beyond routine servicing. Some roundhouses include shop facilities internally or in adjoining buildings.(Wikipedia)