§ 208-e. Extra work by members of certain police forces. Notwithstanding the provisions of any general, special or local law or any rule or regulation of any police department or commissioner or head thereof, any member of a police force of a transit authority with facilities in any municipality may engage in extra work for another employer outside his regular hours of duty for not exceeding twenty hours a week provided that such extra work does not interfere or conflict with his regular duties as a member of the police force of such a transit authority or his availability for emergency duty nor affect his physical condition to the extent that it impairs his ability to efficiently perform such duties and further provided that the type of employment shall first be approved by the appropriate transit authority.