About the light clock thought experiment: Not treating the EMr travelling between the mirrors in the material object clock as a separate category for consideration from what the observers see in their own reference frames (that omission), probably also stems from the original category differentiation error in "On the electrodynamics of moving bodies", Einstein 1905. In which there is no category differentiation between the given methods for train length comparison. Now it can be said, one measurement is of a beable material object, the train itself, (an object not dependent on observation) in space (not dependent on observation), and the other the measurement obtained from observing the seen, observer generated, manifestation of a train in observer generated, seen, spacetime.