I'm very surprised a thread like this hasn't been made yet. I know SC4D has had one for the longest time... but nothing here at ST.
Anyways the guidelines are very simple. Post pics of stuff you're working on. Other memebers can provide pointers and whatnot if you're asking for help/opinions. Otherwise you're just posting content that is in the works/not quite finished.
For example, this is what I was working on today. Click images for full size.
Still a lot of tinkering to do... Maybe I'll insert some rail curves as well.
Anyways you get the premise. Now show us what you're working on!