I have a 15x19 that I started in 2006. I started buying hollow core doors 24"x6'8", and lucked into a batch that were damaged at Lowes for $8 ea. There is also a place nearby called "Broke and Poor" which sells scratch and dents and used doors for anywhere from $5 to $25 depending on size and condition.
As I had time and funds, I built brackets from 1x4s, carefully calculating how many 6 or 8 foot boards would yeild the most brackets for the ammount of money I had to spend that weekend.
As I put up another door, I lad some Atlas flex track and switches, and woked on my track plan.
I had a basic large loop with a yard and industries within a year.
This is actually a great planning aid, as you can run short trains, and modify the plan in real time/space as you go, with the benefit of seeing what fits and works, and what doesn't.
Just when I though my plan was finished and after I'd laid all the track and placed structures (either completed, partailly completed, or mockups), I had a revelation, and revamped 75% of the plan and decided to add a second deck.
this revelation occured in January, and now I'm buying double row shelf standards and prepping the dorrs to be "raised" as I have funds to do so.
This has worked out very well for me, as I have been able to operate by myself continously for quite some time, and have had several sessions with a guest or two on several veaiations of my original plan.
The only downside is the lack of scenery and ground cover, which I can live with if the result is a better more satisfying plan.
The first time I had a friend over to operate, his comment was "looks like the Cat Mountan by David Barrow".
I considered that a compliment.
I an now, however, at a point where I have the option of working on either the doubledecking, the helix, or picking an area of finalized plan and adding scenery.
IMHO, not too bad for 3 yaers and a major plan revision with ensuing construction project, all the while having a fully operational layout.