What part of speech is mainline?

Mainline can be categorized as an adjective and a verb.

Parts of speech

1. mainline is a verb, present, 1st person singular of mainline (infinitive).
2. mainline is a verb (infinitive).
3. mainline is an adjective.

Adjective
mainline - normal, principal or standard.
mainline - Of or pertaining to the principal route or line of a railway.
mainline - Of or pertaining to a surface railway as distinct from an underground, elevated or light rail one.
Verb
mainline - inject into the vein; "She is mainlining heroin"