I’m on a train sitting next to an open window - it’s a bit stuffy - travelling towards a station. A siren is sounding at the station. As I travel toward the siren its pitch is higher that it would be if I were stood on the platform. Once I have passed the siren and I’m moving away, its pitch becomes lower. The reason for this change in pitch, as I understand it, is that as I travel toward the source of the sound it could be viewed that I am stationary but the speed of the siren’s sound has increased by my speed toward the siren. More cycles per second enter my ear (although the wavelength remains the same) and so the pitch of the sound increases. Now if there was a red flashing light on this siren and the train was fast enough (and I was willing to stick my head out of the window of a train travelling at a significant potion of the speed of light) I would also see a frequency increase on the siren’s flashing light. However relativity tells me that I would also measure the speed of that light to be the same as if the train were stationery. Why is this?