That happened to me once. I saved my session with the "Save as" command from the Main Menu. I then closed that session and reopened it with the file name that I just saved. When the route started the buzz was gone. Remember this is a computer and some time things hang. I have found it best to "Save as" when the session is either "Paused" or your consist is stopped. If you don't bad things can happenen. I have had problems that causes the sesssion to derail when it restarts
Now I have a question.
Any hints on how to get to the first station (ECML Lincoln Relief: Trainz 2014) on time. I was able to complete the session once with 100 points, but I switched engines to the freight that caused the slow down and drove it to the siding, clearing the way for my passenger train. I didn't get credit for the session though.
Bob