The Planning Board may authorize, at its discretion, the issuance of a conditional use permit for any of the following buildings or uses in Residential A, B or C Zones: a bus passenger station: and a telephone exchange or other public utility building or structure except a storage yard, a garage or a workshop. Such permits may be issued, however, only upon satisfaction in each instance of such conditions as to general character, height and use of the structure, the provision of surrounding open space and treatment of the grounds and as to street capacity and use, as in the opinion of the Planning Board may be necessary to safeguard public health and comfort and convenience, and as may be required for preservation of the general character of the neighborhood in which such building or structure is to be placed or such use is to be conducted.