The Anacostia and Potomac River Railroad Company, the Washington Railway and Electric Company, the City and Suburban Railway Company, and the Capital Traction Company are hereby permitted to lay duct lines on such streets as may be necessary for the proper operation of their lines, the location of such duct lines to be approved by the Mayor of the District of Columbia, and the cost thereof shall be borne and paid solely by said street-railway companies, and they shall be solely liable for all damages to persons and property occasioned by any construction or work authorized by this section.