mainline
adjective
1. normal, principal or standard.
2. (rail transport) Of or pertaining to the principal route or line of a railway.
3. (rail transport) Of or pertaining to a surface railway as distinct from an underground, elevated or light rail one.
noun
1. (aviation) An airline's main operating unit, as opposed to codeshares or regional subsidiaries.
2. (computing) The main repository for a software project, from which different versions (forks) may be split off.