an observer on a train sees natural phenomena on that train that look the same whether the train is moving or not. The second postulate, constant speed of light, says observers in a train station see light travel at the same speed whether they measure light from within the station or light from a moving train. A light signal from the station to the train has the same speed, no matter how fast a train goes.

In the theory of special relativity, the two postulates combine to change the definition of "relative speed". Rather than the simple