The Coalition for Citizens with Disabilities (CCD) is an umbrella group for organizations supporting the rights of the disabled in the United States. This is from a recent position paper they've put out on service animals:
... Public transportation, as well as Amtrak, must allow any service animal trained to work or perform tasks with the exception of primates and exotic snakes. Any limitations must preserve access for all species and sizes of dogs, cats, rabbits, miniature horses, capuchin monkeys, and other species that can be trained to behave appropriately and be safely brought on public transit and rail. ...


