The last thing you want is to ruin what should be a low stress, highly enjoyable travel experience by not being prepared. If you plan to take a train trip with your dog in France, there are a few things you'll want to bring to ease the journey: For smaller dogs bring a dog carrier. Most of the trains say that ‘small dogs in carriers’ are allowed, but we've often traveled with our dog on a lead without problem. For all dogs bring a muzzle, a water bowl, a bottle of water, a few bags and some paper towels for cleaning up accidents, and a blanket or towel for your dog to lay on.
When you order or purchase your tickets be sure to tell the train ticket agent that you are traveling with a dog. For some trains, there is a small charge for traveling with a dog, and a ticket will be issued allowing your dog to board the train with you. When arranging the itinerary be mindful of how long your dog can go without a toileting break. There are some really wonderful overnight trips throughout France that wouldn't be appropriate for most dogs because the ride lasts for 10 hours or more without any significant stops. Also remember that if your dog is nervous or sensitive, traveling may make him require more frequent potty breaks.
If you are traveling with a larger dog it is smart to ask if there are coupes available; if your dog is particularly large this may be a requirement. Smaller dogs can be held or lay between the seats; it is a mistake to allow them to lay in the aisle because people will be passing often and your dog could get hurt.
No matter how enamored you are of your dog, don't expect other people to share your sentiments. After all, they don't know your dog. My experience is that most other passengers will not interact with dogs on the train, though a few will offer treats or a pat. If you've traveled with a dog in a less dog friendly place, you will welcome this lack of attention (as will your dog. )
Sydney and her dog, Geena, have traveled in Greece, Italy, and France. Sydney writes about her nomadic life on her blog: http://www.exoticmundane.blogspot.com