(aviation) An airline's main operating unit, as opposed to codeshares or regional subsidiaries.

(rail transport) The principal route or line of a railway.

(fishing) In longline fishing the central line to which the branch lines with baits are attached.

(plumbing) The pipeline carrying wastewater to the public drains or a septic tank.

(slang) A principal vein into which a drug can be injected.

(computing) The main repository for a software project, from which different versions (forks) may be split off.

(chess) Alternative form of main line

(slang) To inject (a drug) directly into a vein.

(by extension, informal, transitive) To consume voraciously.

(computing, transitive) To integrate (code, etc.) into the main repository for a software project, rather than separate forks.

Normal, principal or standard.

(rail transport) Of or pertaining to the principal route or line of a railway.

(rail transport) Of or pertaining to a surface railway as distinct from an underground, elevated or light rail one.

(chess) Of a sequence of opening moves: being part of a main line ("a standard sequence of opening moves considered to be best play").