A bi-articulated bus or double-articulated bus and sometimes train-bus, tram-bus, trackless tram or double bendy bus is a type of high-capacity articulated bus with an extra axle and a second articulation joint, as well as extended length. Bi-articulated buses tend to be employed in high-frequency core routes or bus rapid transit schemes rather than in conventional bus routes.

An articulated bus, also referred to as a slinky bus, bendy bus, tandem bus, vestibule bus, stretch bus, or an accordion bus, is an articulated vehicle, typically a motor bus or trolleybus, used in public transportation. It is usually a single-decker, and comprises two or more rigid sections linked by a pivoting joint (articulation) enclosed by protective bellows inside and outside and a cover plate on the floor. This allows a longer legal length than rigid-bodied buses, and hence a higher passenger capacity (94–120), while still allowing the bus to maneuver adequately.