1. **State the problem:** We need to find the diameter of a subway token given its radius is 12 millimeters. 2. **Formula used:** The diameter $d$ of a circle is twice the radius $r$. This is expressed as: $$d = 2r$$ 3. **Apply the formula:** Substitute $r = 12$ millimeters into the formula: $$d = 2 \times 12$$ 4. **Calculate:** $$d = 24$$ 5. **Answer:** The diameter of the subway token is 24 millimeters. This means the token is 24 millimeters wide across its center.