Oh, thanks, I didn't think of that..... Kinda dumb

Perhaps a hint in the clues via TAB at some point would be nice?

So that I ran out of belt, and had to change my design to include more undergrounds because I didn't have enough Belt is not intended? If you run out of one type of belt, you're out and you can't use it any more. Then you need to click Start & Back to get more.

I recently played the new Belt Routing Scenario from 0.18.
An interesting part of the challenge is that the amount of given Belt is limited.
The Problem is, after placing your belt, starting the level and then going "back to building" you go back to the starting amount of belt, gaining extra belt ...