I suspect this has been covered before, but as it was a nice day to take some photos, (and we all know how you folk love pictures,) I decided to post my findings.
I splurged at the weekend and bought an Accucraft flat car:
Just like the one in this "manufacturers photo", mine has a upward curve, somewhat more pronounced. Jeff Redeker, who has a few of them, was talking about putting his in the oven with a brick on top. I figured I should put it in the sunny window of my office, but then I remembered I have a floor heater along the front of the window. So the heater got turned on, and I grabbed a few heavy bits - my drill, a piece of marble from a bathroom, and a box of zincs waiting to be installed on my boat. I took the trucks off the flat and supported the end beams using some 1x1s and rubber 'feet'.
It worked! How's that for a sway-backed flat car....
HOWEVER, I discovered the truck bolsters had been significantly damaged by the heat - and it turned out to be the truss rods, which actually work on this car. The following picture shows the bolster (top) and the ends of the trusses that I have bent flat in an attempt to fit them under the bolster.
The truss rods had a right-angle end behind the bolster. Here's the other end of the car, with the truss rods loose, because there is no bolster at the other end of the car.
Finally, here's the turnbuckle, which is threaded. You'll note the unpainted brass where I unscrewed the truss in an attempt to make it go back under the bolster.
I guess I'll have to solder something on the end of the rods to make them fit - seems too much of a hassle to unscrew them all.
I'll post a photo of the finished flat when I get it back together again....