Tugs and Barges

[Exocet]

Once upon a time I was told that tugs pulling barges will trail a line off the last barge. I am told they do this in order to retrieve the barge in it breasks loose while towing. Now this all seems resonable, and I always leave a wide berth when encountering any vessels, but is this true. Also were would I find out what the lights above the tugs mean? While towing I usually see 3 white lights, one above each other. What do the lights mean, if they pull only one barge, is there only one light?
Thank for the reply...Exocet