What age is a senior on Amtrak?

Could you please clarify for me what the specific definition of a "senior" passenger is on Amtrak, and if there's a designated age range or criteria that determines eligibility for senior discounts or promotions? I'm interested in understanding the policies surrounding senior travelers on Amtrak's trains and if there's a standard age threshold that is typically recognized. Thank you for your assistance in clarifying this matter.