1Railways (chiefly US) (originally) either of the metal plates connected to or supporting a rail of a point, and attached at one end to the rails of the main track; (later) either of the metal plates reinforcing each tie at a point, and running under the rail brace and part of the rail itself (now chiefly historical).
2Any of various plates on or under which a switch may be mounted; especially a flat cover for the electric wiring of an outlet box.
Origin
Mid 19th century.
Definition of switch plate in US English:
switch plate
noun
1Railways (chiefly US) (originally) either of the metal plates connected to or supporting a rail of a point, and attached at one end to the rails of the main track; (later) either of the metal plates reinforcing each tie at a point, and running under the rail brace and part of the rail itself (now chiefly historical).
2Any of various plates on or under which a switch may be mounted; especially a flat cover for the electric wiring of an outlet box.