The following paragraph is from [Morin's *Special Relativity: For the Enthusiastic Beginner*](http://bayanbox.ir/view/2067467042440500165/Morin-Special-RelativityForBeginner.pdf) (page 40, the *'rear clock ahead'* section, remark number 6):

> 
> What if we have a train that doesn’t contain the above setup with a light source and two light
> beams? That is, what if the given events have nothing to do with light? The $\frac{Lv}{c^2}$ result still holds, because we could have built the light setup if we wanted to (arranging for the light-hitting-end events to coincide with the given events). It doesn’t matter if the light setup
> actually exists.
> 
> 
> 

I don't see how *we could have built the light setup* proves that the rear clock ahead phenomenon happens without the light setup. There is no way we can know if this phenomenon we are witnessing is only due to our space changing its rules, i.e. conforming to the presence of light.

Is there a more formal and comprehensive explanation of why the relativistic phenomena still occur 'when there is no light in the train'?