Nighttime In The Switching Yard Lyrics

Lyric discussion by smartalek 

Cover art for Nighttime In The Switching Yard lyrics by Warren Zevon

I read somewhere -- zero clue where -- that this song was supposedly written about heroin. Anyone have any idea whether there's any basis at all in reality for that? I'm just not seeing the connection (as it were) -- unless it's punning on "the mainline" and "the tracks"??