Ошибка компилятора CS0120 - C#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
 
namespace ConsoleApplication4
{
    public abstract class Vehicle
    {
 
    }
    public abstract class Train : Vehicle
    {
    }
    public abstract class Car : Vehicle
    {
    }
    public class Compact : Car, IPassengerCarrier
    {
    }
    public class SUV : Car, IPassengerCarrier
    {
    }
    public class Pickup : Car, IHeavyLoadCarrier
    {
    }
    public class PassengerTrain : Train, IPassengerCarrier
    {
    }
    public class FreightTrain : Train, IHeavyLoadCarrier
    {
    }
    public class DoubleBogey424 : Train, IHeavyLoadCarrier
    {
    }
    public interface IPassengerCarrier
    {
    }
    public interface IHeavyLoadCarrier
    {
    }
    class Program
    {
        string Show(Vehicle inside)
        {
            return inside.ToString();
        }
        static void Main(string[] args)
        {
            DoubleBogey424 TrainType1 = new DoubleBogey424();
            FreightTrain TrainType2   = new FreightTrain();
            PassengerTrain TrainType3 = new PassengerTrain();
            Pickup CarType1           = new Pickup();
            SUV CarType2              = new SUV();
            Compact CarType3          = new Compact();
            Console.Beep(100,100);
            Console.WriteLine(Show(TrainType1));
            Console.ReadKey();
        }
    }
}