Here are a couple of lumber loads that I did to fit specific cars. This one, on a Walthers GSC 52'6" flatcar, is hollow. The stacks are removeable, so I marked the interior of each noting the car-type that they fit.
This one, also hollow and removeable, is built to fit an Athearn 50' flatcar. The flatcars have had the plastic deck removed and replaced with basswood strips; this had the added benefit of lowering the car slightly, so it hunkers down over its trucks. The bulkhead ends are parts taken from the Walthers cars in the first picture.
This load of hydro poles was constructed from wood dowels, tapered and then "grained" with coarse sandpaper. All of the poles are full-length, as the wide and narrow ends alternate. The technique was outlined in an article in RMC. This load is also removeable, and fits only the Proto2000 gondola, due to the need for working drop ends. The idler car is a Walthers GSC, although any flatcar would work.
These loads of machinery, being pushed into GERN Industries, are made of parts from the scrap box. The "bodies" are extra panels from a Walthers 89' trilevel autorack, with a few added on parts from MDC. I also added some scratchbuilt lifting tabs so the "machinery could be loaded/unloaded by an overhead crane. Each assembly is "welded" to a "steel" plate, which in turn is "bolted", through the blocking, to the deck of the car. In reality, the loads are removeable: the car closest to the loco is another Walthers GSC flat - its load, with the attached "wood" blocking (styrene), simply lifts out of the stake pockets. The tiedown rods are steel music wire, with the uppers ends bent and inserted (but not glued) into the lifting tabs, while the lower ends simply sit loose in the appropriate pockets.
The depressed centre flatcar is a cast metal car from Authenticast, and is from my very first HO trains. Each piece of blocking, both under the load and at the lower ends of the tiedown rods, sits on a strip of double-sided carpet tape. The placards on the stakes and blocking are "DO NOT HUMP" warnings.
The following sequence of shots depicts a multiple car shipment of a large, overhead, electric crane. The crane parts are built from styrene shapes and sheet and all are removeable. Each is marked, on the bottom, with the car type that it fits, and in the case of the flatcar loads, also the particular stake pockets to be used. The gondolas are from Proto2000, while the flatcars are more Walthers GSC's. Because of their size, cranes like this are assembled on-site. There would be other large parts on open cars, plus some smaller components in boxcars. The first gondola, 4582, is a spacer between the locomotive and the train, and is empty.
The blocking is styrene, with nbw detail added, and the blue signs are advertisements for the fabricator, Dominion Bridge.
Here's an easy load to model: an empty, but not yet cleaned, car. Depending on its next lading, it may not get cleaned, either. Left on the car floor is dirt (real), some gravel, a few lumps of coal, and lots of scrap lumber. The lumber was blocking under the load, or stakes to hold it in place, or dunnage, used between the load and the stakes to prevent shifting. Train crews would often scrounge this stuff for kindling for caboose or section house stoves. The scrap lumber is scraps of basswood that was laying around, some was left almost "new", while other pieces were stained to represent pieces that had been in the car a while. Like all of my loads, the junk is loose.
I have lots of other loads too, but no pictures available. They include many types of scrap (most protoype scrap shipped has been sorted, as to type of metal, size, or even particular alloy, as steel mills can be very fussy as to their requirements). Also coal and coke, gravel, steel plate and, eventually, machinery. All loads are loose material where appropriate and all are removeable.
Wayne