Yep, I understand your post. To a layperson like myself, I was just fascinated about that one small part of the thought experiment and didn't quite understand its explanation. I'm no longer a student - I was many years ago - and just wanted clarification about that one part.
I've always been curious about this thought experiment, especially about what the platform observer "sees" as the train passes by him traveling at a velocity close to the speed of light and at the same instant the passenger shines a light inside the train car that bounces off a front mirror...