[Gedankenexperiment (Thought Experiment) 1]

A given train's length is 600,000 km and it is on a uniform motion. The lines of vision of people inside the train are fixed within the train and outside scenery is ignored. Let me explain the following phenomenon from the standpoint of those inside the train.
The lamp placed in the center of the train glitters and the light travels in both frontward and rearward directions.

A second later, the light reflects off the front and rear walls.

Two seconds later, the reflected light reaches the center of the train and people inside the train must have observed that "the front and rear walls glittered simultaneously and the light from the two directions reached at the same time".

[Thought Experiment 2]

Let's suppose this train sweeps away in front of an observer at a speed as fast as a quarter of the speed of light (Uniform Motion). Please see to it that "the speed of light is 300,000 km/sec." Let me explain the following phenomenon from the standpoint of those outside of the train.
The lamp placed in the center of the train glitters and the light travels in both frontward and rearward directions.

4/5 seconds later, the light reflects off the train's rear wall.

4/3 seconds later, the light reflects off the train's front wall.

8/5 seconds later, the light from left reaches right in front of an observer. The observation will be: "The train's rear wall glittered. The light from left reached me".

8/3 seconds later, the light from right reaches this observer. And the observation will be: "The train's front wall glittered. The light from right reached me."

[Conclusion]

If "the speed of light is constant" is used as the base, when relative velocity is different, then such phenomenon that "the incidents happening simultaneously inside the train" becomes "the incidents are not happening simultaneously when looked at from outside of the train" will occur. The physical phenomena do not coincide. In other words, it may be wrong to assume that "The time scale is absolutely the same for anyone".

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