I have a very limited space in which to build my layout, roughly 9' X 11'. The space is closed on three sides, so I was pretty much restricted to a U-shaped arrangement. To get maximum use of the space, I decided to go with two levels, with a helix connecting them. I would have loved to go 24" or larger radius, but it just didn't work. Instead, I used 22" radius on the helix, and 24-26 everywhere else.
I was surprised to find that I could run almost anything on that, including a Broadway GS-4. The only problem was the overhang of the loco hits the vertical spacers on the helix. I'm going to re-work those by moving the vertical supports outboard of the sub-roadbed, and I should then be able to run that engine all the way up and down the helix without hitting anything. I've also run an SD70ACe and a couple other 6-axle diesels on it without trouble.
The one thing that doesn't work, though, is just how silly those large engines look on a tight radius. They have a very 'toy-train' appearance to them, so I will stick with my original plan of only running 40' freight cars and B+B diesels. I will only roll out the big iron for special occasions.