The track builder is cool and all, but right now you can't really share your tracks, nor challenging times from other players. There's not that much you can do with it. An ingame repo/forum section ingame with times from other players would really be a nice addition and would make the Track builder more complete and used. There's a lot of things possible with this!
Hey! While there is currently no repo for track or time sharing, you can manually share track builder tracks. You can find your saved tracks under C:\Users\(You)\Documents\BeamNG.drive\trackEditor. You can also put shared tracks in this folder and they should appear in your track list in the track builder (might need to restart the game for this)