The ornamental design for top rail corner piece, as shown and described.
FIG. 1 is a perspective view of a top rail corner piece showing my design (with environment shown in phantom and not claimed as part of the invention);
FIG. 2 is a top plan view thereof;
FIG. 3 is a front elevational view thereof (left side view is a mirror image thereof);
FIG. 4 is a rear view thereof (right side view is a mirror image thereof); and,