Things get really unintuitive when you go near the speed of light. Einstein’s “Special Relativity” is describing that. Watch a couple of videos on the topic. It’s mindbending but seriously cool.

In short: The speed light is always constant FOR EVERY OBSERVER. That means, if you would hold a flashlight in a very fast moving train, the light would travel as the same speed for you as for a stationary person that is watching your flashlight from outside the train.

But how could that be? Aren’t you “adding” the trains speed to your flashlight? So shouldn’t the light in your train travel faster in your train? Or maybe slower? No. Light speed is always constant - but what is NOT constant is space and time. It is relative to the observer. Time and space can stretch/dilate to make up for what seems to be a paradox. E.g. your trains would shrink in length the faster you go. But it would look different to you than it does to an outside observer.

As I said, it’s mindbending, but there are a couple of cool and simple videos on the internet to get a better grasp on the matter.