Happy Birthday! New York City's first official subway system opened on October 27, 1904. Small dogs can cruise the Big Apple inside carriers on the subways, trains, and buses, while on the San Francisco MUNI buses and railways, dogs of any size are welcome as long as they are well-behaved, muzzled, and it's not during a major commute time.

Rules about pets (excluding service animals) and mass transit vary by city and state so, if you're thinking of traveling with a pet, check with your local transportation agency first. However, even by many animal lovers, pets on buses, trains, and subways can get the cold shoulder – how do you feel?