Every NG car starts out life as a flatcar. Every flatcar I've seen worked on has a steel bolster. It's basically a long bar or piece of plate that stretches the width of the car. On that plate sits a receptacle for the truck, which has a corresponding receptacle (one's male, one's female, but I don't recall which is which). Also on the plate is a casting where tension rods terminate, which go up over the innermost sills (and through the intermediate sills).
Coach 9 seems to be slightly different, in that there's a wood component to the bolster. Basically the bolster is a lot beefier (more than just a plate).
Whether some of the older cars started out in the 1800s with wooden bolsters, I don't know. I'd be surprised if they did, because steel's a lot stronger and the car builder's craft had advanced along 50+ years since the beginning of railroading.