If the button is on, when the player starts dragging a track the level of the first square is taken as base level to the rest of the dragged track.
If impossible to level land => error message and incomplete track is built.
This should be easy to implement since it's based on already existing features.
This is specially handy for diagonal rail tracks, it's very annoying to level land for those

At first I though on adding diagonal land level features, but that wouldn't be that easy to implement. I think the auto-level is much handier and can improve gameplay a lot.
Edit: ok I found it here now. Auto-leveling would still be faster though.
If toolbars are getting too big, I would remove the "toggle remove track" button, I see it as an advanced tool that is done with Ctr only. For beginners the demolition one is fine. Signals still need the remove button but that may be set on the signals toolbar only.
(Sorry if already suggested, I didn't find it)
Cheers
G