The ornamental design for a top rail assembly for a crib or bed, as shown and described.
FIG. 1 is a top perspective view of a top rail assembly for a crib or bed showing my new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a front elevation view thereof; and,
FIG. 4 is a side elevation view thereof.
The subject matter shown in broken lines is for illustrative purposes only and forms no part of the claimed design. The bottom and rear view (not shown) form no part of the claimed design.