A uniformly moving train passes by a long platform. Consider the events ‘engine crossing the beginning of the platform’ and ‘engine crossing the end of the platform’. Which frame (train frame or the platform frame) is the proper frame for the pair of events?
As the platform is stationary it is considered as the proper frame for this event.
The train travels with a speed which is very less as compared with the speed of light. So, it does not violate the second postulate of relativity.
Thus, both train and platform are proper frame.