Just a dumb four wheeler wondering what I saw.

I'm not now and never have been a professional driver, but I have been commuting to and from Atlanta for the past 20 years or so. I worked as a field service tech for 6 of those years travelling all over the state to work onsite. I work in logistics now, so I rely on you guys every day and appreciate the work you all do, mostly very thanklessly done. Many of you have gone above and beyond to save my hide on more than one occasion and I am very appreciative. Ran across this trailer on I-85 on the way home yesterday evening. I don't recall ever seeing equipment like this and it intrigued me. The metal box on the back appears to be on rails running the length of the trailer, but the tracks are on the side edge and not on the surface of the deck. My google-fu has let me down on this one, does anyone know what this is used for?