If the light clock was on top of the train, it wouldnt be in a inertial system. If a table tennis game was played inside the train, the ball would move as if played on the platform. But it wouldn't be possible ontop of the train. Does the light react the similarly to a table-tennis ball or is a photon too small to be affected? So would the observer on the platform see the light trail behind the train? If the light clock theory doesn't work or is effected in some way (i'm not a brilliant scientist so i have no clue what would happen), what would happen if the train, after some time of travelling, was suddenly upgraded into a open-patio-class seating carriage i.e. the roof and walls of the train was suddenly ripped off? Will the time difference just suddenly dissappear?