In Ralph's case, he manually carries some cars that he wants to photograph as virtually interchanging with his or other layouts, puts them in the diorama, and photographs them to publish on his blog. Other modelers have taken this a little farther and had a mini or micro layout that, while not connected to the main layout, operates as a "branch". Cars are manually carried as needed between the main layout and the subsidiary micro.
About 10 years ago I was inspired by a micro layout plan at the late Carl Arendt's site, Box Street, designed by the prolific Jack Trollope:
I realized I had enough scrap items laying around from my main layout construction that I could build exactly that plan. The was the preliminary result:
At first I had fun taking it outside and photographing it in natural light, but it was still a hassle to carry it in and out with rolling stock, set it up. plug it in with an extension cord, run it, take it down, etc.
But after a while, I decided to mount it on shelf brackets in a storage room next to my layout room:
Eventually I set it up as a two-station "branch" of my main layoyut on JMRI. The interchange from my main layout is at West Egg. A "route" in JMRI called Hand Carry hosts a "train" called Manual Carry that instructs the 0-5-0 transfer run which cars to move from the Box Street layout to West Egg and vice versa:
A separate JMRI job creates a switchlist for the industries on the Box Street layout (called Paper Box on the LF&NW -- the main industry is a paper mill). One advantage is that it gives me extra spots for car types like wood chip and pulpwood cars that I don't have enough spots for on the main layout.
This is still on DC, not DCC, so I can use locos that I haven't yet converted. However, I've made enough progress on conversions that I'm now thinking about running the DCC bus through the wall to this layout and making it convertible from DC to DCC via a DPDT switch. You can see the Bachmann train-set DC controller that it now uses. A DCC controller will be better for regular operation, but I can still switch to DC for testing new locos or occasionally running unconverted ones.
Then, maybe extending it along the wall. . . I need more spots!