Presently there is no way to download a track without adding it to a playlist. I would like to be able to download my inbox while I'm at home (on wifi) and play the tracks in the car. It would be even cooler if I could toggle an option and have my mobile application download all of my "Unseen" inbox tracks when I'm home (on wifi) and undownload them after I've "Seen" (listened to) them (or not, more options).
This idea has been submitted more than 1 year ago but unfortunately hasn't gained enough kudos during that time (10 kudos/year). In order to keep an overview of the active ideas in this forum, this idea will be closed for now. Note that this does not mean the idea has been declined by Spotify.
Please feel free to re-post this idea if you still feel you want this implemented. With a little change in presentation, maybe this time it will gain the necessary support from the community.