Therefore the wheel separation - and the transverse dimensions of train and track - must be invariant, the same for all free-float observers moving along the track. (If you think that the actual trans...Therefore the wheel separation - and the transverse dimensions of train and track - must be invariant, the same for all free-float observers moving along the track. (If you think that the actual transverse contraction might be too small to cause a wreck for the train shown, assume that both the wheels and the track are knife edges; the same argument still applies.)
