Oh hi and sorry for the late answer...
My tagging of surface=none is based on the german https://wiki.openstreetmap.org/wiki/DE:OpenRailwayMap/Tagging#Gleis 
As it seems to appear a bit more regularly atm that these questions here are asked I had a look into the english version (even if I'm completely unsure in which order those pages were translated, I would think that the german one could have been the "original" in this case?) and theres nothing about surface=none, so I understand the confusion... 
Yet the tagging is as well put into the josm tagging presets. So maybe it would be a better option to discuss this issue in a bigger round.. Yet I'm a bi confused as I thought that I had mapped more places than just in Braunschweig with this and I cant find them in overpass.
From the wiki "none" should mean a gravel like "normal" surface