Cal. Public Utilities Code § 1202

The commission has the exclusive power: (a) To determine and prescribe the manner, including the particular point of crossing, and the terms of installation, operation, maintenance, use, and protection of each crossing of one railroad by another railroad or street railroad, of a street railroad by a railroad, of each crossing of a public or publicly used road or highway by a railroad or street railroad, and of a street by a railroad or of a railroad by a street. (b) To alter, relocate, or abolish by physical closing any crossing set forth in subdivision (a). (c) To require, where in its judgment it would be practicable, a separation of grades at any crossing established and to prescribe the terms upon which the separation shall be made and the proportions in which the expense of the construction, alteration, relocation, or abolition of crossings or the separation of grades shall be divided between the railroad or street railroad corporations affected or between these corporations and the state, county, city, or other political subdivision affected. (d) To authorize on an application-by-application basis and supervise the operation of pilot projects to evaluate proposed crossing warning devices, new technology, or other additional safety measures at designated crossings, with the consent of the local jurisdiction, the affected railroad, and other interested parties, including, but not limited to, represented railroad employees.