The Medicare Access and CHIP Reauthorization Act (MACRA) of 2015 mandates the removal of social security numbers from Medicare cards. As a result, every person with Medicare coverage will be receiving a replacement card displaying a new Medicare number that is unique to them. This change will better protect your identity. Railroad Retirement Board will mail you a new Medicare card by June of 2018.
The Centers for Medicare & Medicaid Services (CMS) issued a press release covering this initiative in greater detail (09/14/17):
Chose the following link to view a sample of the RRB Medicare Card.
In the meantime, beware of anyone who contacts you about your new Medicare card. Medicare or the RRB will never ask you for personal or private information in order for you to receive your new Medicare number and card.
Report all instances of fraud or potential fraud to 1-800-MEDICARE.