It's inevitable that many of us simply run out of layout building area, and we either then start building up or down utilizing 3rd dimensional space, re-build, or take over an upstairs or attic space 
Others may join a club and rent a large space. And then, there are the movers and shakers of our hobby, who do the impossible, and expand into another basement 
You may have heard of Howard Zane, a long-time avid HO modeler (and he also builds and sells models in O scale) and his model railroad is unbelievable, as are his models - which are up there in cost. I have a couple of DVDs of his layout. He's expanded his basement twice! In one interview with Allen Keller, he said model railroaders are like sharks; they have to keep building less they die 
. Most of us will never expand our basements; it's a significant cost and frankly, moving into another home with larger area is probably less expensive, though we will have lost much of our previous work which is defeating the expansion concept.
This is an older article I found that partly explains his passion:
Model Railroader on Track With His Dream - The Washington Post
Those long bridges are awesome - what are they made from or are they from a custom builder?
