I have seen plenty of photos of tarped loads on flat cars, usually covering equipment. However, I can't recall seeing a gon with a tarp. I'm certainly not saying this photo is unique, but given the number of photos I have seen, this one stood out. It would make an interesting and relatively easy thing to recreate. I'm leaning towards a microfiber cloth dyed to simulate the canvas tarps of the late Steam Era. I think it's worth a try. When I do it, you'll see it here.
The gon is PRR 337254, a GRA, reweighed in March, 1950 at the Rose Lake, Illinois, eastbound shops on the Pennsy.
The subject of the photo was a New Haven I-4 Pacific in the Hartford Yards, but this is far more interesting, so I zoomed in and cropped to show all of the gon that I could. All three cars can be modeled in HO. The gon can be replicated using the Westerfield kit, the Texas & Pacific automobile car can be built using an MDC single sheathed auto car (read about how in Prototype Railroad Modeling, Volume 3) and the Pere Marquette auto car can be built from Speedwitch kit K111 (wow! two Speedwitch plugs in one sentence!)