It was in the terms and conditions of what I received. You may not have that limitation going for Select Exec, here is the language
Terms and conditions:
Earn double Tier Qualifying Points (TQPs) per qualifying Amtrak trip from October 15, 2014 12:00:00am CT through December 31, 2014 11:59:59pm CT. Minimum spend of $35 per one-way segment is required for travel to qualify. Limit of two qualifying one-way segments per day. A maximum of 1,000 bonus Tier Qualifying Points may be earned through this promotion. Tier Qualifying Points (TQPs) are the base points you earn on your Amtrak travel and are the points that count toward earning tier status. This offer is not transferable, and only eligible members may participate. Must be an Amtrak Guest Rewards member and register for this offer online at AmtrakGuestRewards.com/FastTrack to participate. Members must include their membership number when making reservations for travel to qualify. Other terms and conditions may apply.