The present invention is the ornamental design for the cover for rail attachment. The nature and environmental use of the claimed design is as follows:The cover may be a plastic, wood, or metal cover, through plastic covers are presently preferred. A plastic cover may be formed from an extruded material, such as vinyl. The cover has an opening that receives an end of a rail. The cover hides of covers the attachment of the rail to a support post.

FIG. 1 is a perspective view of a cover for a rail attachment according to the present invention;

FIG. 2 is a top view of the cover for a rail attachment;

FIG. 3 is a of a first side of the cover for a rail attachment;

FIG. 4 is an upside-down view of a second side of the cover for a rail attachment;

FIG. 5 is a view of a third side of the cover for a rail attachment; and,

FIG. 6 is a view of a fourth side of the cover for a rail attachment.

Ordinarily, the bottom of the cover will not be visible when the cover is in use as a cover for a rail attachment.