switch(FEAT_ROADVEHS, SELF, power_switch, current_roadtype) {
  ELRD: return 1200;
  default: return 690;
}

item(...) {
  property {
     road_type:                  ROAD; 
     power:                      690 hp;
     ...
  }
  graphics {
    power: power_switch;
    ...
  }
}