An airline's main operating unit, as opposed to codeshares or regional subsidiaries.
"After years of flying on regional carriers for their routes, passengers finally saw American Airlines expand its mainline fleet into the small towns where it previously only operated through codeshare partners."
To inject (a drug) directly into a vein.
"After years of struggling to find money for pills, he finally managed to mainline heroin from a needle shared by someone else in the alleyway."
Normal, principal or standard.
"The company decided to focus its marketing budget on the mainline product rather than the experimental prototype."