Where different completion dates for accessible stations are established under this part for a station or portions of a station (e.g., extensions of different periods of time for a station which serves both rapid and commuter rail systems), accessibility to the following elements of the station shall be achieved by the earlier of the completion dates involved:

(a) Common elements of the station;

(b) Portions of the facility directly serving the rail system with the earlier completion date; and

(c) An accessible path from common elements of the station to portions of the facility directly serving the rail system with the earlier completion date.