I believe the plastic thing might be the assembly that simulates the car bolster and the air bags that provide the secondary suspension on the prototype, with the outside bracket that anchors the longitudinal radius rod that stabilizes the truck frame on the real thing. Ed's picture of Amfleet Coach2 above shows how this is modeled... the support, bar, and simulated truck bolster end are all one solid piece completely separate from the modeled truck frame.
While it might be amusing to model the air springs out of soft material (like some diaphragms) I think the truck swing is far too great for 'prototype' detail there... although you might use a disc of that clear plastic to make 'extended bearing area' fore and aft of the model bolster for the bags to slide on, even if that's just the opposite of prototype swivel action...
What's the consensus here on rods that visibly anchor truck frames or provide support? On some classes of electrics the secondary springing actually twists to give the truck pivoting, which might be fun to see modeled...
I think if the bracket is what's hurting the truck swing, it might be cut off, the 'rod' shortened where it joins the fake bolster, and then reattached. That might reflect only a couple of scale inches, and might not cause too much rivet-counter grousing...