Two or more snowplows plowing a multi-lane highway in a specific fashion where the one in the innermost lane is just a few metres ahead of the next one, and so on, ensuring snow is bumped from the inside lane right on out to the outside shoulder.
Also known as "echelon plowing."
"It took me hours to get home. I got stuck behind a congo line."
"We've had 6 cm of snow within an hour and the congo lines are already out."