Cal. Pub. Util. Code § 100165

The district may enter into agreements for the joint use of any property and rights by the district and any public agency or public utility operating transit facilities; may enter into agreements with any public agency or public utility operating any transit facilities, and wholly or partially within or without the district, for the joint use of any property of the district or of such public agency or public utility, or the establishment of through routes, joint fares, transfer of passengers or pooling arrangements.