The area is a storage depot with a back room that is initially broken into. Leaving this room leads to some stairs up into the main storage area, with upper catwalks overlooking a train car area. The floor is soaked with oil that may be ignited if a firefight ensues at the end of the quest The Big Dig. There is also a power armor station on the lower level.
The NH&M Freight Depot only appears in Fallout 4.