Odds of rolling boxcars
Both dice showing six.
Chance per roll
2.78%
1 of the 36 ways 2 6-sided dice land add up to 12 — about one roll in 36.0. Among the 11 totals 2d6 can make, that is the 2nd least likely of 11 — tied with 1 other, since the distribution is symmetric.
Every total 2d6 can roll, with 12 picked out. The peak is 7 at 16.67%.
The 1 way to make it
Every one of them, which is the whole proof of the figure above — count the rows, divide by 36.
Other rolls worth knowing
Need a pool that isn’t here? The calculator takes any number of dice with any number of sides. If you are asking because of a board, what the 12 is worth in Catan is a different question with a different answer.