Hello PF. Re. the light clock / train thought experiment. It seems to me that the clock calibration chosen, ( light's mirror to mirror path ), is determining the outcome of the experiment. And that a different calibration would give a different result. e.g. assuming the mirrors are one metre apart, calibrate the clock to tick over each time light travels one metre. One metre is the same measure in both frames as the distance between the mirrors can't change, and, the speed of light is constant. You still have the perpendicular and diagonal paths, but they are now measured in one metre lengths. The passage of time would therefore be the same in both frames.