One opportunity is for non-fighting vehicles to be loaded on flat cars. Military units rely on an extensive fleet of motor vehicles, many of them with no fighting capability at all. Among these are trucks, various kinds of trailers, and Jeeps (if not modified to carry weapons). Large trucks have been shown in a previous post here, on a Richard Hendrickson model, at: https://modelingthesp.blogspot.com/2015/09/more-richard-hendrickson-open-car-loads.html . Here is a repeat of that photo, loaded on a railroad flat car (Illinois Terminal) and blocked for loading according to the relevant AAR diagram:
In addition, Richard modeled a load of fuel trailers. These are in fact Roco U.S. military trailer models, but in the unmarked state could well represent military surplus equipment or even civilian copies of the same designs. They are loaded on Santa Fe Class FT-V flat car 91506.
Still another choice for military loads is non-vehicular ones. Here again, there are lots of possibilities. My choice was a artillery piece, the Army’s 8-inch field howitzer, model M2A1, widely used in both World War II and Korea for its excellent accuracy in the field. Though not excessively heavy, a gun like this made a large load and might be found on a variety of types of flat car. There is a nice Roco HO model of this piece, which I acquired years ago when it was still in production (they can still be obtained on eBay from time to time). The model even comes with a limber, used for towing. My model has attached blocking of the type seen in World War II photos.
A gun like this might well move on an Army USAX flat car, like the Roco model I have been showing in this series of posts, but was light enough (one source says 17 tons) that it could easily be moved on a typical 50-ton or 70-ton railroad flat car, as you see in this photo of a Westrail NP flat car. (You can click on the image to enlarge it if you wish.)
At this point I want to mention another resource for modeling. In the previous posts, I have cited several good books about prototype armored vehicles, but there are also extensive resources on modeling. To choose just one example, I like the softbound book shown below, a collection of articles from Fine Scale Modeler magazine (it’s a Kalmbach book from 1995). It almost entirely describes large models, 1:35 scale, and not all models are American armor, but it contains lots of fascinating details one could add to HO scale armor, including Shermans and half-tracks.
Finally, at the Cocoa Beach, Florida meeting last January (my post about that meeting is here: https://modelingthesp.blogspot.com/2019/01/cocoa-beach-meeting-2019.html ), I photographed a very well-done model by Larry Lawler of a Tank Recovery Vehicle or TRV. This too was built on an M4 Sherman hull. He placed it on a reworked Roco flat car like mine, though both his armor markings and the flat car represent a later era than I model, as does the type of blocking shown, But it makes a terrific military load.