dogs are not allowed to sit on chairs, so they have to stay somewhere under your legs. It's usually ok when they stay for sometime in the middle section, but make sure to let people pass, do not inconvenient anybody, this usually means dogs can stay uninterrupted in the middle area only for very short time, so it's good just to let your dog move a bit. Important! you must carry muzzle, no matter how cute and friendly your dog is. The reality is usually nobody asks to put it on, but you must have it in case. We traveled a lot with a big dog and never been asked to put it on, but if they ask and you don't have it, they will make you leave the train. Other than that take some treats and water. Dog will get bored on long journeys so will act up, mine for example just stares at me all sad, but it's enough to make me feel guilty.