The ornamental design for an endcover of a window curtain rail, as shown and described.
FIG. 1 is a perspective view of an end cover of a window curtain rail;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a perspective view of the design showing that one end cover is mounted at one end of a window curtain rail. The broken lines in the drawing illustrate environmental structure and form no part of the claimed design.