I used to get SO angry over that go. I can't handle incredibly hard puzzles although, theres nothing better then watching a train/car go along a bridge while it slowly collapses behind them ;)
I went to the Website, but you have to buy it, and it is with trains and not cars, is there anyway to get the version shown? (it says it used to be freeware, but now is retail....remind you of something that you also build in?)
I forget exactly how you do it. You switch to the mortar as you're going prone or something. This was in v0.2 or something so I'm sure it's fixed by now.