Application shall be made by the interested public utility by written petition containing a concise statement of the proposed transaction, the reason therefor and such other information as may reasonably be required by the commission. Upon the filing of such application, the commission shall promptly investigate the same, with such hearing and upon such notice as the commission may prescribe, and unless the commission shall find that the proposed transaction is unlawful or is inconsistent with the public interest, it shall give its consent and approval in writing.