infrastructure.mss

#rail {
  line-width:2;
  line-color: black;
  [zoom >=6] {
    ::line1 { line-color:#999; line-dasharray:4,4; }
    ::line2 { line-color:#000; line-dasharray:0,4,4,0; }
  }
  [zoom = 5] {
    ::line1 { line-color:#999; line-dasharray:4,2; }
    ::line2 { line-color:#000; line-dasharray:0,4,4,0; }    
  }
}

#roads {
  line-color:#999;
  [zoom <= 4] {line-width:0;}
  [zoom  = 5] {line-width:0.25;}
  [zoom  = 6] {line-width:0.5;}
  [zoom >= 7] {
    line-width:2;
    line-color: black;
    ::outline {
      line-width:1;
      line-color: #999;
    }
  }
}