Ошибка при создании документа XML

 public class XML
    {
        const String pathXmlFile = "transport.xml";
        private XmlSerializer formatter;
 
        public XML()
        {
            formatter = new XmlSerializer(typeof(List<object>));
        }
        public void Serialize(List<object> listOfTransportFirst)
        {
            using (FileStream fileStream = new FileStream(pathXmlFile, FileMode.OpenOrCreate))
            {
                formatter.Serialize(fileStream, listOfTransportFirst);
            }
        }
        public List<object> Deserialize()
        {
            using (FileStream fileStream = new FileStream(pathXmlFile, FileMode.OpenOrCreate))
            {                
                   var listOfTransportSecond = (List<object>)formatter.Deserialize(fileStream);
                    return listOfTransportSecond;                
            }
        }
    }

    [XmlInclude(typeof(Trains))]
    [XmlInclude(typeof(Balloon))]
    [XmlInclude(typeof(Ships))]
    [XmlInclude(typeof(Cars))]
    [XmlInclude(typeof(UnderWaterTransport))]
    [XmlInclude(typeof(Aircraft))]
    [DataContract]
    [KnownType(typeof(Trains))]
    [KnownType(typeof(Balloon))]
    [KnownType(typeof(Ships))]
    [KnownType(typeof(Cars))]
    [KnownType(typeof(UnderWaterTransport))]
    [KnownType(typeof(Aircraft))]
    [Serializable]
    public class Transport
    {
        [DataMember]
        public int IdTransport { get; set; }
        [DataMember]
        public int MiddleSpeed { get; set; }
        [DataMember]
        public int NumberOfPassengers { get; set; }
        [DataMember]
        public int CostTravel { get; set; }
        [DataMember]
        public string Name { get; set; }
        [DataMember]
        public int MaximumTravelDistance { get; set; }        
        public Transport()
        {
 
        }
        public Transport(int idTransport, int middleSpeed, int numberOfPassengers, int costTravel, string name, int maximumTravelDistance)
        {
            IdTransport = idTransport;
            MiddleSpeed = middleSpeed;
            NumberOfPassengers = numberOfPassengers;
            CostTravel = costTravel;
            Name = name;
            MaximumTravelDistance = maximumTravelDistance;
        }
 
    }