A highway or town way may be laid out across a railroad previously constructed when the governing body of the county adjudge that the public convenience and necessity require it and, in such case, after due notice to the railroad corporation and hearing all parties interested, such body may thus lay out a highway across a railroad or may authorize a city or town, on the petition of the mayor and aldermen thereof, to lay out a way across a railroad in such manner as not to injure or obstruct the railroad.