It shall be the duty of any railway company within the city to construct, at its own cost and expense, safe, suitable and convenient crossings, culverts and bridges where its tracts intersect any public street or thoroughfare in the city, upon being notified in writing by the director of the department of public works so to do. Such notice shall prescribe the time within which such crossings, culverts and bridges shall be constructed, which time shall be as to the director of the department of public works may appear reasonable, equitable and just. In cases of unavoidable delay, and after satisfactory proof of the cause thereof has been made by such railway company, the director of the department of public works may extend the time for making such improvement for such a period as may appear reasonable, equitable and just.
(1964 Code, § 31-5)