You are correct, I should have said there was an atomic clock out the window that the walker looked out at.

https://www.quora.com/What-if-you-walk-forward-on-a-ship-moving-at-light-speed#%3A~%3Atext=You+would+experience+nothing.%2Cof+travel+wouldn't+exist.

Because of relativistic effects, from your point of view on the train you would just walk forward. But you would notice a strange effect while the trains were accelerating: your atomically synchronized wristwatch the clock you can see out the window has slowed down and stopped counting time. So it seems that your journey to the front of the train takes no time at all.

From someone standing on the side of the tracks catching a glimpse of you and the train as you whizz by, the front of the train is moving at light speed. You’re at the back of the train completely frozen still, unable to move forward because the front of the train is moving away at light speed.

Weird things happen when you’re talking about the limits of physical reality.

Sound is air vibration

Sound is not exclusive to air, it can be generalized to vibrations in any media. Whale song and dolphin echolocation are certainly sounds, and we’re almost always talking about them propagating in water rather than air.

which has to travel from one place to the next

No, that isn’t how sound works. In air this would be a description of wind, not sound.

just transfer kinetic energy to the adjacenct atom

This is actually a good description of how sound waves propagate.