I was gonna keep it quiet for a bit, but i've seen that other people have already figured out how to do it too. So here you go, and enjoy:
This is how i figured it out:
1. All you have to do is drop an object into the scene, then another one of that type near by (for tutorial purpose we'll use an empty train carriage).
2. Move to one of the objects with your corona and press edit to pick it up.
3. Now you're in edit mode, you're able to place the carriage on top of the other one. (Some objects are fussier than others, and may take a bit of moving around or fiddling to drop into an acceptable spot, but you get used to it, and do it quicker once you know each object's placement difficulty).
That's all there is to it.
You can create stacks miles high, you can create extreme ramps which go from the road into the sky, over skyscrapers. And you can even delete from underneath leaving you with floating platforms if you wish. Your only limitation is the 50 props and the item limits.
Now go make something original with it and post it back here!