The ornamental design for a railing supported planter, as shown and described.
FIG. 1 is a top front right side perspective view of a railing supported planter showing my new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a front side elevational view thereof; the rear side elevational view being a mirror image thereof;
FIG. 4 is a left side elevational view thereof, the right side elevational view being a mirror image thereof; and,
FIG. 5 is a bottom plan view thereof.
The broken lines in FIG. 3 represent environment and form no part of the claimed design. The broken lines in FIGS. 4 and 5 represent portions of the planter and form no part of the claimed design.