When, in the opinion of the selectmen of any town or of the common council of any city, a footway upon the line of any railroad bridge or causeway within the limits of such town or city would be of public convenience and the railroad company owning such bridge or causeway does not consent thereto, such selectmen or common council may call out the Commissioner of Transportation, who, after due notice to such company, shall inquire into the facts, at the expense of such town or city. If the commissioner finds that a footway along such bridge or causeway would be of public convenience, he shall authorize such town or city to construct or maintain the same at its own expense and to attach the same for support to such bridge or causeway. Such footway shall be constructed entirely outside of the bridge or causeway to which it is attached and so constructed, maintained and used as not to interfere with the use of such bridge or causeway.