class Car extends Vehicle {
  constructor(speed, cost) {
    super(speed);

    var c = Symbol('cost');
    this[c] = cost;

    this.intro = `This is a car runs at
      ${speed}.`;
  }
}

interface TrainProps {
  speed: number;
  cost: number;
}

class Train extends Vehicle {
  constructor (trainData: TrainProps) {
    super(trainData.speed);

    var c = Symbol('cost');
    this[c] = trainData.cost;

    this.intro = `This is a car runs at
      ${trainData.speed}.`;
  }
}
