Yes, greensound works on a plane or train, in a browser, with an internet connection.
Does it work offline?
No, not to start. Loading the page, and fetching a real recording like jungle rain or beach waves, both need a connection.
Headphones on a plane: how loud?
The same guidance as anywhere on this site: loud enough to notice, quiet enough that an announcement or someone speaking to you still gets through. The listen safely note below has more detail.
Which sound suits engine noise?
This page does not rank one noise color as suiting engine noise better than another. White noise spreads equal energy across the whole range you can hear (its own page has the detail), rather than favoring the low end the way brown noise does; green sits toward the middle. Any of the three is one tap away to compare.
Where the sound comes from
White noise. Made in your browser, not a recording.
Listen safely
This is a sound, played back through whatever speaker or headphones you already own. It doesn't diagnose, measure, or promise anything about how you sleep, focus, or feel — it's just noise and a handful of real recordings, mixed the way you like them.
Keep the volume low enough that you'd still notice a smoke alarm, a phone ringing, or someone at the door. If you fall asleep in headphones, choose a volume and a pair you'd be comfortable wearing all night, and a session length that ends when you'd want the sound to stop.
If you use this near a baby, keep the speaker several feet from the crib, never inside or clipped to it, and keep the volume low — the same care you'd take with any sound machine in a nursery.
If your relationship with sound, or with sleep, feels difficult rather than simply restless, a doctor or audiologist is the right person to talk to. This is a mixer, not a clinician.