I was working as a conductor on a special operation for a group of railfans on the Niles Canyon Railway and we were doing a runby at Farwell Bridge. One of the patrons set himself up on the edge of the ties between a signal mast and the mainline. It was bad enough that he was on the edge of the ties but if didn't pay attention to the mast and ran into it then he could pinball himself back in front of the train. I politely asked him to move to a safer location and got an earful about how he was taking pictures before I was born and HE knew how to take pictures safely. I then pointed out to him that I didn't care how long he had been taking pictures and that he was standing in an unsafe location and needed to move. All I asked him to do is step to the other side of the signal which he ended up doing. He still got a good picture and didn't give the engineer a heart attack. If he continued to argue with me my backup plan was to stop the train and tell everyone in the photo line that he was the cause of the runby being cancelled.