Cal. Pub. Util. Code § 105101

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, wholly or partially within or without the district, for the joint use of any property of the district or of the city, public agency, or public utility, or for the establishment of through routes, joint fares, transfer of passengers, or pooling agreements.