WhitePass' structure will fit on some depressed center cars and his high capacity flat car is great.
You have already found some construction equipment and there is a lot more that can fit some DP and high cap cars.
A couple of my favorite loads
<kuid:174941:1033> 15m Steal Box Beam - commodity - and <kuid:174941:1035> 15m steal box beam - scenery - by dylanviey available here
http://uslw.net/downloads.php?action=view&id=292
<kuid2:58223:41282:1> Concrete Bridge Beam Construction 01 Product by ish6 on the DLS
Ben Dorsey has made dozens of these High Capacity cars and they are all on the DLS. Figuring out how to assemble them can be a challenge!
Many have a single attachment point on the center floor and you can edit the config.txt to use it but using PEV's attachment maker you can add attachment points at the center but level with the upper decks, off center on the center floor, and on the upper decks. Great for nudging a crane or transformer into the right position or adding a pallet of oil drums or stack of lumber.
Searching for "transformer" will find an interesting selection and making large loads for these could be a interesting project for the budding modeler.
I've been meaning to approach kenichiro about the possibility of him/her separating out elements of his/her Industrial Plant series of assets to be used as loads but I keep forgetting.
And, of course, there are davesnow's harvesters which were specifically designed for DP cars and will fit on some of Ben's cars too.