Both the rail cap and the rail base should be constructed of a hardwood. Together this will provide a solid mounting area for the cushion. Soft woods will give when the cushion is struck by a ball, but solid hardwoods will not, giving the table a livelier playing characteristic. Furthermore should the cushion ever need to be replaced, a soft wood usually chips and becomes unsuitable for regluing when the old cushions are removed. In addition to beauty a hardwood rail cap provides the strength that will hold in the feather strip.
Softer woods tend to spread over time allowing the cloth to pull out and become loose.
Loose cloth can cause the cushion to separate from the rail and produce sluggish play. A hardwood rail base is ideal to hold the cloth staples. Softer woods can chip, split and crack after multiple staplings. Hardwoods will not, allowing your table to be recovered many times over. Diamond sights vs. round sights.