Cal. PUC § 7807

Where the portion of the street is occupied by a track or tracks of a different gauge from the track or tracks proposed to be constructed thereon by a line of street railway under a different management, the latter street railway may nevertheless construct its track or tracks, subject to the provisions of Section 7806, over the same ground as is occupied by the prior track or tracks, if they can be so constructed as not to interfere with the operation of the prior track or tracks beyond such necessary interference therewith as is incident to such construction with reasonable skill, care, and diligence.