This is still the underside. As we move towards the front of the unit the gap between the underside and rails becomes small so the width of the black stiffening piece is reduced to fit between the rails. I slightly misjudged the reduction point and had to file two slots in the leading edge of the stiffener (upper left).
At the point where the railer goes below rail level, at the narrowing of the white part (lower right), I filed the edge of the plastic thin above the rails to get a snugger transition to the railheads. I realised later that this is not neccessary because the wheels still roll on the flanges at this stage.