Mainline meaning : An airline's main operating unit, as opposed to codeshares or regional subsidiaries. The main repository for a software project, from which different versions (forks) may be split off. To inject (a drug) directly into a vein. To integrate (code, etc.) into the main repository for a software project, rather than separate forks. Normal, principal or standard. Of or pertaining to the principal route or line of a railway. Of or pertaining to a surface railway as distinct from an underground, elevated or light rail one.