I was recently on an Amtrak train when the lady across from me had brought her dog (had to be a 9 pound dog!!!). The conductor was not happy when he saw the dog sitting next to her on the seat. He told her she had to put it away and not let it be seen. We were wondering what he would do given that the ticket wording says no dogs!....this was a trip from Boston to NYC!
If your dog is quiet, you could probably get away with it. I have great carriers...no one even knows I have a dog! They look like pocketbooks....people are shocked when I tell them my precious dog is inside of it!!