That was my first thought too. I’ve seen a couple of them, although I can’t really place where I’ve seen them.
In Rochester, PA, there’s a retaining wall built of stacked hopper cars.
Well I’ll be darned! I was searching for bridge supports and found that a flatcar bridge kit is available.
“Engineered” without respect for the ‘niceties’ of actual engineering, at a surprisingly high price for something you could make with a train-show flatcar and some rudimentary stock in the first place.
Flatcars were designed and stressed to take loading only on their bolsters. It defies belief to put a brace in the middle of the stream that just goes against the sideframe girders in the middle. Most of these bridges did have simple abutment support right at the “car ends”, but that was just preparing the banks with simple fill or retaining structure and laying the car on them at minimum expense.
The California modular system needed temporary bents to function as intended – as a quick-erecting system in field conditions with equally-rapid knockdown when no longer needed. That will almost never be something that a model railroader would want to use, unless they wanted to use the original Livermore idea of cutting up flatcars for all the pieces of the bridge support (!)
Just west of the town center in Williams AZ they’ve used an bright red (repainted?) ATSF Boxcar for a covered walkway over a concrete wash.