There also is the option of getting a cab at the station. Short ride, but not free like the shuttle bus shuffle. Saves anywhere from 20 to 45 minutes depending on wait times at the station and airport.
You have to take the shuttle to the airport and then shuttle from airport to car rental. Some car rentals will pick you up from the station if you call them.
Think you have to go to the airport then to Amtrak
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