Idaho Code § 62-306
Whenever a state or county highway crosses or shall hereafter cross a railroad at grade, the railroad company shall at its own expense construct and maintain that portion of such highway between the rails and for a distance of not less than two (2) feet outside the outer rails. The crossing shall be planked or surfaced with other suitable material for the full width of the traveled way, including shoulders, and shall be maintained at all times in a smooth and firm condition. Where a public agency having jurisdiction of the highway crossing the railroad wishes to have the crossing surfaced with material of higher quality, the public agency and the railroad company may agree that the railroad company install the material and that the additional cost, over and above the cost of the railroad company's standard installation, may be paid for by the public agency with public funds.
Idaho Code § 62-306