The other month I was on a 3-4 day train trip from Brisbane to Adelaide. I wouldn't describe the track sounds as soothing - struggled to sleep the first night, and wasn't much better the next. It's not overly quiet and you get jolted around a fair bit when travelling at speed. Maybe if the extreme parts of the soundscape were moderated a bit though.
The other night my wife was using a soundscape to get to sleep - ocean noises and a creaking hull of a timber ship. The ocean noises were fine, but the periodic cracking timber had me wide-eyed and annoyed until I figured she was asleep and turned it off. Sounded like someone's bones being broken.