Pub. Util. Code, § 7813

The right to lay down a track for grading purposes and to maintain it for a period not to exceed three years may be granted by the governing body of any city, city and county, or county, but no such track shall remain more than three years upon any one street. The track shall be laid level with the street, and shall be operated under such restrictions as not to interfere with the use of the street by the public. The governing body may grant the right to use steam or any other motive power in propelling the cars used on the grading track, when public convenience or utility demands it, but the reasons therefor shall be set forth in the ordinance, and the right to rescind the ordinance at any time reserved.