Washington Department of Transportation Rail Director Ron Pate says previously only service animals were allowed the on the trains.
Reservations, which cost $25 per pet, must be made in advance and will be accepted for travel beginning March 7.
Officials say traveling pets must be odorless and harmless, not disruptive and require no attention during travel, among other stipulations.
Each train can take up to five pets.
Pets are not allowed on trips to Canada or trips longer than seven hours.