re-reading I think I get your problem whats confusing you
I am observing that beam closing with and striking the passenger at c+v!
That side on stationary observer position we are talking is the GOD view with no speed of light to us.
That is your so called zero frame in relativity and it does not exist for any actual observer because there is speed of light to us.
And I think thats may be where your coming unstuck now.
Einsteins K frame is the Zero reference frame because we have to be able to describe the movements on a common system because everyone will see something different. Thats not an actual observer like you or I, it's godmode so we can describe the movements.
If we were side of to that scene we wouldn't see it like that at all in real life.
WHY => There is speed of light to us as the side on observer we would actually see the whole scene different and thats why we have to create a zero frame.
Where your speeds where slow like with trains etc you put an observer there and they would be able to see almost what a zero frame looked like but not with stuff moving near the speed of light. Because the time of light to observer would change what they saw.
So what I should correctly say is you can deduce a c+v speed in a zero frame (Einstein K frame) but you could never directly observe it.
As I said we have deduced the edges of the universe must be moving away from each other faster than the speed of light and we have deduced two galaxies are moving away relative to each other faster than the speed of light.
As per post above speed of sound is no different it's only ever one speed. Stand on a moving car and yell and the sound still moves at the speed of sound but what happens is the pitch changes to people stationary on the ground.
