As the motorcycle is neither a bus nor a train it is not included in the exceptions. As it is designed to carry a (or at least one) passenger - having a passenger seat and footpegs for passengers, it is also a passenger vehicle.
Although some states may not require passenger facilities on a motorcycle, I know that Maryland is one of the states that specifically require a passenger seat and footpegs as they recognize that a motorcycle is a passenger vehicle. Believe me, I have looked at both Federal and State laws about this.