Discussion on: Casting stinks. Generic classes are worse.

public interface IEngineer
{
  //Engineer Properties
}

public class LeadEngineer : IEngineer
{
  //Lead Implementation
}

public interface IAssemblyLineInstructions { }
public class SedanAssemblyLineInstructions : IAssemblyLineInstructions {}
public interface IVehicle {}
public interface IAssemblyLine {}

public class AssemblyLine
{
  private IVehicle _vehicle;
  private IEngineer _enginee;
  private IAssembyLineInstructions _instructions
}

public interface IFactory 
{
  public IAssemblyLine { get; }
}

public interface INamedFactory : IFactory
{
  string FactoryName { get; }
}

public abstract class VehicleFactory : INamedFactory
{
  public string FactoryName { get; set; }
  public IAssemblyLine AssemblyLine { get; set; }
}

public void Print(INamedFactory factory)
{
  Console.WriteLine($"Welcome to {factory.FactoryName}");
  Console.WriteLine($"The Engineer on duy is {factory.AssemblyLine.Engineer.Name}");
}

public class SedanAssemblyLine : IAssemblyLine
{
  //Assembly lines have engineers
  public IEngineer Engineer { get; set; }
  public SedanAssemblyLine()
  {
    // Set defaults in the constructor, not using generics.
    // This assembly lines engineer is a SedanEngineer
    Engineer = new SedanEngineer();
  }
}

public interface IFactory
{
  public IAssemblyLine AssemblyLine { get; }
  public void PrintEngineer();
}

public abstract class Factory : IFactory
{
  public IAssemblyLine AssemblyLine { get; set; }
  //All factories will do this, in the same way, so Don't Repeat Yourself
  public void PrintEngineer()
  {
    Console.WriteLine("The engineer is ${AssemblyLine.Engineer.Name}");
  }
}

public class SedanFactory : Factory
{
  public SedanFactory()
  {
    //Sedan Factories have Sedan Assembly Lines... Obviously....
    AssemblyLine = new SedanAssemblyLine();
  }
}