"Monikers, also known as Streaks or Hobo art is a form of graffiti. Monikers are the names or drawings on the sides of a freight car on freight trains. Monikers are produced with a solid paint stick, industrial crayon, or a lumber crayon. Monikers serve the purpose for a moniker artist to gain fame or recognition, or to share stories with others. Many moniker artists have a unique design they produce, and sometimes write the area they are from, or date that the moniker was produced. Many of the artists are train hoppers, and railroad workers."

-From Wikipedia.

Other Subreddits worth checking out:

/r/Vagabond

/r/Bombing

/r/Trains