It's as simple as the titles says, the click-clack sound of trains going over the gaps between rail sections (note: this is at a 39-foot (~12 meters/blocks) intervals, so maybe the frequency of the sound could be dependent on speed?)

It's just kind of eerie when you're standing next to a moving train and there's no sound at all from it (and the engine is too far to be heard)

This already exists. If you are playing in newer versions of minecraft, the sound system may be slightly bugged

Maybe depending on locomotive age, the sound can change (suppose "recent locomotives are on recent tracks so there's no more gaps and no more click-clack sound")

There is that kind of sounds presently, depends on track/stock I believe and speed.