Here's the answer I got from The US RRB about box 12:
BOX 12 - MEDICARE PREMIUM TOTAL - This is the total amount of Part B, C, and/or D Medicare premiums deducted from your railroad retirement annuity payments during the tax year indicated on Form RRB-1099-R. This amount is for informational purposes and only applies to tax years 1997 and later. If your Form RRB-1099-R is for a tax year prior to tax year 1997, this box is blank even though you may have had Medicare premiums deducted from your railroad retirement annuity payments. Medicare premium refunds are not included in the Medicare total. The Medicare total is normally shown on Form RRB-1099 (for U.S. citizens) or Form RRB-1042S (for nonresident aliens). However, if Form RRB-1099 or Form RRB-1042S are not required for the tax year indicated, then the Medicare total will be shown on Form RRB-1099-R. If your Medicare premiums were deducted from your social security benefits; paid by a third party; and/or you paid the premiums by direct billing, your Medicare total will not be shown in this box item.
