The speed of light is equal to 299 , 792 , 458 m s^ -1

According to the postulates of the special theory of relativity, the speed of light in vacuum is a universal constant.

It is independent of the relative motion of the source and the observer. Therefore, the speed of light is the same for all observers in inertial frames, regardless of their state of motion.

Thus, the speed of light is $299{,}792{,}458 \text{ m s}^{-1}$ relative to a moving train on the earth, relative to the sun, and relative to the earth.