main line1 
noun
1A chief railway line: [as modifier]: a main-line station...
- The Alcan company shipped its bauxite from Port Esquiville, a short distance from Bodies, utilising the existing main-line railway.
- At the same time, 24 major projects were successfully achieved and BR became the only profitable main-line railway in Europe.
- Virgin main-line trains made additional stops, too.
1.1North American A chief road or street.This applied particularly to the main lines of turnpike road constructed before 1811....
- Each main line busway passenger must pay Rp 2,500 for a one-way trip.
- At first I was riding the main line through the braking bumps, and the suspension, as always, was still super smooth.
2 informal A principal vein as a site for a drug injection.
verb
(mainline) [with object] informal Inject (a drug) intravenously: Mariella mainlines cocaine five times a day [no object]: he started mainlining on heroin...
- Her performance electrifies the story of the model who died of the virus she probably picked up by mainlining heroin.
- It begins with the great detective, Sherlock Holmes, mainlining on a 7% solution of cocaine.
- She had mainlined morphine, but never more than 10 cc.
Derivatives
mainliner
/ noun ...- Except for age, there are significant differences between mainliners and fundamentalists on all independent variables.
- But the mainliners are quiet - and their numbers are diminishing so fast, they're not sure they'd be heard if they screamed.
- Most mainliners do not view modern culture as inherently inimical to religious belief.
Main Line2
A popular name for a series of affluent suburbs west of Philadelphia in Pennsylvania, along the old Pennsylvania Railroad main line.