I'd say it's a rather obscure Easter Egg. It's nothing crazy, rather it's simply a Halo Assault Rifle partially buried in some rubble. I love little nods like this in videogames. I guess Bethesda has a few Halo fans amongst its ranks.
I found this just half an hour ago, but I wanted to share it before anyone else discovered it. For those who want to know, I found this by following the train tracks south until I nearly reached the end of the map.
I also provided two different links in case one doesn't work for whatever reason, but they both show the same thing.
[url=https://youtu.be/dQw4w9WgXcQ]https://youtu.be/FdxR7logxXA[/url]
[url=https://youtu.be/dQw4w9WgXcQ]http://xboxdvr.com/gamer/Pureeey/video/12783187[/url]