A roll of two 6s (the highest roll possible) on a pair of 6-sided dice.
The probability of rolling boxcars in a single roll of two dice is 1/36, or 2.777...%.

In order to have a 50% chance of obtaining at least one boxcars in rolls of two dice, it must be true that

(1)

so solving for
gives

(2)

In fact, rolling two dice 25 times gives a probability of