What is this rear bumper for?
 

So I just purchased a 2014 GMC 3500 HD Regular Cab WT and it was obviously some sort of fleet truck as the mileage is very low for its age, only 56,000 miles. It's a 6.0 litre L96.

It has some odd grates along the top of bed sides and an odd rear bumper. Any idea what this rear bumper would be used for? Railway truck? Something else?

I've attached a picture. Thanks!