Mushroom design is just another tool in a track planner's potential arsenal for increasing the amount of layout you can get into a given space.  It's similar to dual level in some ways, but it requires more engineering, so it's not an easy way to get more layout.  It also effectively divides your layout into separate areas by virtue of the different floor heights.  With traditional dual level construction, you're just stacking more layout on top of what you already can do, but the mushroom borrows space from your existing level and mulitplies it by about 50% maybe and nests the new part within the old in a way which makes it invisible to the rest of the layout.  If you look at Joe Fugate's Siskiyou line, he basically has two layouts in his space, the mainline and the Coos Bay branch.  Except in the transition area by the helix, you can't see one line from the other.

Joe has written that he likes the idea mainly because both levels of the layout can be seen from near eye level, which he feels is the best height for appreciating a layout, which is true in many ways.  I, on the other hand, have nothing against dual level construction or seeing trains anywhere from 36 inches off the floor to as high as 60 inches; I might even go higher under the right circumstances.  I have combined the virtues of dual level with mushroom construction to produce a design with as many as 6 levels stacked on top of each other, but only 2 can be seen at a time from any specific aisle:This is a very extreme example of what can be done.  Whenever you start looking at multi-floor designs, you have to be very careful; we have several construction professionals involved in our club, so everything is engineered safely:  basically, don't try this at home, kids. Nevertheless, it does show how effectively you can use the vertical space to maximize how much layout you can get in given floor area.

The thing I like most about the mushroom is that separation of the various parts of the layout.  When a crew is up on the branchline at McKerrow or Espanola, they are in a different world from Sudbury Yard, even though the yard is only a few feet away.  As we continue our construction the various parts of the mainline become ever more distant from one another, and not just from the point of view of a train getting from place to place; there's a lot of walking to do in our building already and there will be more before we're done.  Where a traditional design would have given us maybe 4 aisles, we have 9; even a more typical mushroom would have yielded at least 7 aisles.  It gives you a lot of bang for the buck.

Mark Lestico's layout is terrific, but it is a level layout that simulates a climbing railroad with scenery. Very different from mushroom style layouts that must, by definition, include some climbing to get one deck above the other (unless the decks are independent and unconnected physically, which is also possible). If one is really interested in understanding the Mushroom concept, study will reap dividends.

Joe Fugate's articles in the January and February 1997 issues of Model Railroader are a great source. Another very interesting article on the topic is Henry Freeman's article on the late Jerry Bellina's mushroom style plan in Model Railroad Planning 2003. John Armstong's design for Otis McGee's SP Shasta Route in the April 2005 MR may also be helpful.

These articles are probably most instructive about mushroom style layouts in longer and narrower spaces. John Armstrong's earlier mushroom-style layouts are also interesting, but are set in "square-ish" spaces (and look more like the classic "mushroom" shape that inspired the name).

I wonder what the fire marshall would have to say about such a closely packed mammoth of a mouse-nest design...

Nice work!!

I did some dabbling with mushroom design.  The nice part is you can use the areas under the intermediate floor for storage for the first floor, for things like bookcases, totes, and other storage.

The pain is getting up form one level to the next.  my personal goal is no helixes.  The nice aprt of the mushroom design is that I was able to get the upper deck to come down 6" below the deck level before starting into the grade module, and then the lower module could prodive an additional 12" of rise towards that upper level, the combined effort reducing the necessary grade dissapation of the grade module by 18"! 

My best work thus far yielded a rather long grade Module of sorts that alone fit into a space 9'x18' or so - that's with 30-36" curves and only one crossing of one line over the other.  Still, that's a lot of space!!

I wonder what the fire marshall would have to say about such a closely packed mammoth of a mouse-nest design...

The owner of the property was a volunteer fireman and had the whole brigade out to see the place.  There were no concerns raised over the layout, since it was considered a private club.  If we were a public display there would have been other requirements, no doubt.  We've also had insurance inspectors out and the only concern there was that one wanted a fireproof shield mounted over a wood burning stove since the wall angled over top of it a bit.  No problem.  We use over-code wiring and installed a second floor fire escape, plus exit signs and emergency lights, none of which were required by any authorities so far.

And it's been called a rabbit warren before, but never a mouse nest...