mainline definition
Wiktionary (0.00 / 0 votes) Rate this definition: mainline (Verb) To inject (a drug) directly into a vein. mainline (Adjective) normal, principal or standard. mainline (Adjective) Of or pertaining to the principal route or line of a railway. mainline (Adjective) Of or pertaining to a surface railway as distinct from an underground, elevated or light rail one. Origin: * main + line