Cal. Pub. Util. Code § 101200

The district may enter into agreements for the joint use of any property and rights by the district and any city, public agency, or public utility operating transit facilities and may enter into agreements with any city, public agency, or public utility operating any transit facilities, either wholly or partially within or outside the district, for the joint use of any property of the district or of such city, public agency, or public utility, or the establishment of through routes, joint fares, transfer of passengers, or pooling arrangements.