adj

• (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").
  "After 3 e5 the Czech Benoni Defense is created. Unlike the mainline 3 e6, which aims for tactical counterplay in the center, Black aims for solidity there. He will choose later what flank to attack on."

verb

• (slang) To inject (a drug) directly into a vein.
  "The patient skinpopped for 1½ years and then started to mainline heroin."
• (by extension, informal, transitive) To consume voraciously.
  "Last month, she lamented that her busy campaign schedule had kept her from mainlining Amazon’s new “Rings of Power” series."
• (computing, transitive) To integrate (code, etc.) into the main repository for a software project, rather than separate forks.
• (transitive) To include (a prisoner) in the general population of a prison.

noun

• (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.
  "Many plumbers will tell you they don't go off property since that is the local water company's responsibility. Most local water companies would prefer that, if a line is snaked, it is snaked all the way to the mainline connection."