No public road, highway, or street shall be constructed across the track of any railroad corporation at grade, nor shall the track of any railroad corporation be constructed across a public road, highway, or street at grade, or shall the track of any railroad corporation be constructed across the track of any other railroad or street railroad corporation at grade, nor shall the track of a street railroad corporation be constructed across the track of a railroad corporation at grade, without having first secured the permission of the commission. This section shall not apply to the replacement of lawfully existing tracks. The commission may refuse its permission or grant it upon such terms and conditions as it prescribes.