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Re: Truss Rod Cars
Can't find a good isometric drawing of a wood car frame, but this picture might help:
It shows metal draft arms on a D&RGW narrow gauge gondola. Note the prominent springs of the Cardwell draft gear. The body bolster, still to be installed, fits in the notch behind the draft arms and the sub sills attached to the bottom of the center sills.
Since railroad cars usually lose their draft gear from a hard coupling impact, the combination of the metal draft arms, bolsters, and sub sills fitted tightly in between the bolsters transmitted buffing forces straight through the car to the other end, and supposedly the next car in line. The problem is, they don't do anything for heavy draft (pulling forces) which is why they were superseded by full steel sills during the thirties.
More photos and commentary here: