Stop.cs

public abstract class Stop
{
    public int StopId { get; set; }
    public StopStatus Status { get; set; }
    public int Sequence { get; set; }

    public void Arrive()
    {
        if (Status != StopStatus.InTransit)
        {
            throw new InvalidOperationException("Stop has already arrived.");
        }

        Status = StopStatus.Arrived;
    }

    public void Depart()
    {
        if (Status == StopStatus.Departed)
        {
            throw new InvalidOperationException("Stop has already departed.");
        }

        if (Status == StopStatus.InTransit)
        {
            throw new InvalidOperationException("Stop hasn't arrived yet.");
        }

        Status = StopStatus.Departed;
    }
}