Several years ago somebody brought a "comfort animal" into the diner and insisted on keeping the dog in his lap during the meal. We insisted that the dog had to be kept below the level of the table top because there were other passengers sharing the table. The dining car was entirely full, and we did not have the option of moving anybody to another seat. The dog's owner complained, and Management decided we had handled it wrong. Interestingly, they never said what we should have done instead, and never showed us a definitive rule to guide our actions in the future. Go figure.
True service animals are well trained and well behaved wherever they are. Pets are entirely different, and home-trained "service animals" are unpredictable. I loved the real service animals I encountered, but I know of situations where untrained animals created messes, were disruptive, behaved poorly, barked, and even bit passengers and employees.
By the way, I'm a dog owner. He's my good friend, but I don't pretend he's a service animal.
Tom