I've only just begun reading about relativity, and am finding Einstein's popular level book "Relativity: The Special and General Theory" quite satisfying. This particular example, from Section X (ON THE RELATIVITY OF THE CONCEPTION OF DISTANCE) has me a bit confused, though: 1 the first measurement being the distance between the points A' and B' on the train, judged from the train as reference body, simply by the "repeated application of the measuring-rod." Any hints as to why it is "by no means certain" that the distances would be the same? I'm trying to picture the setup of the example, but am having a hard time. The definition of time of an event was given as "the reading (position of the hands) of that one of these clocks2 which is in the immediate vicinity (in space) of the event." 2 "clocks of identical construction placed at points D, E, and F of the railway line (co-ordinate system)" which were started with identical settings, simultaneously. I'm picturing something like an A clock and a B clock, which are stopped as points A' and B', on the train, pass by, and would have identical settings (identical times) when stop. Is that even close? I guess you'd need plenty of clocks. In any case, the distance between the could be measured, as he says, but why wouldn't they necessarily be the same distance apart as A' and B', as measured from the train as reference body?