Hi, L.,
I read several books on child rearing and apparently, (unbeknownst to me at the time), kids don't really WANT a yes or no answer. They want to understand that you understand what they are saying. So, if he says, "Watch trucks?" DO NOT answer, "no, I can't now" because he doesn't want to hear that. Instead, you should say, "oh, you want to watch trucks? Yes, I know! You want to watch trucks." Usually, that should be enough to stop him from repeating himself. You don't even need to say that he can't. If he can't watch it for some reason, then you can grant him his wish in fantasy. Such as, "I wish you could watch trucks, too! If the DVD player wasn't broken we can watch lots and lots of trucks!"
It sounds really weird, but try it. I tried it and IT WORKED LIKE MAGIC!!! That is all they want: acknowledgment that you understand what they want. I think many parents (like myself) try to explain that "no, you can't" and we end up with kids that sound like broken records! When he says, "train! Train!" he is waiting for your answer. Simply saying "yes" is not enough for kids at this age. You have to talk in full sentences for them, since they cannot. Next time, when he says that, try answering with, "yes, I see the train! There's the train!" That should also stop him from repeating himself. He is probably repeating himself to get these responses from you, but since you are not saying them, he keeps repeating himself. My 3.5 year old does that, too, until I realize that I have not answered the way the book encouraged me to answer. Once I remind myself that I need to address him verbally instead of just nodding my head or saying "yes" then the repetition goes away right away. Even if he can't have something, instead of arguing about it, I just say to him, "yes! I wish you could go, too! That would be so much fun!" The book made this comparison: suppose you were admiring a dress in the store. How would you like it if you said, "Oh, what a nice dress!" and your husband's reply was, "You can't afford that dress! It is too expensive and we have more important bills to pay." it would probably put you in a bad mood. It would be nicer of him to say, "I wish we could get you that dress. You would probably look fabulous in it and you could wear it to that party we are going to." So even if you don't buy the dress, you could dream about it. So, dream about the things you son also wants and keep him happy with his dreams! Good luck!