function changeMStation(){
  var sel_s = document.getElementById('city_subway_s');
  var sel_m = document.getElementById('city_subway_m');
  var city = document.getElementById('city_select');
  if (city[city.selectedIndex].value == 1){
    sel_s.style.display = 'block';
    sel_m.style.display = 'none';
  }
  else {
    sel_m.style.display = 'block';
    sel_s.style.display = 'none';  
  }
}