You could put in some sort of door.

Maybe a door that slides up when trains are running and has a seal on the bottom to keep things out the rest of the time. Or maybe one that swings from or slides to the right. Mice love hiding spots. I would not do a tunnel.  I would definitely have a way to seal it up. 

Mirror trick

On the finished side of the wall, mount two vertical guideways to hold a mirror flush against the wall as a barrier. Use a plastic one so that you can easily notch it to snug around the rails. Attach a foot bridge to the top of the mirror to serve as a handle for sliding it up and down, and disguise the ends of the bridge, the edges of the mirror, and the guides with trees and foliage. Maybe add a micro switch or two to prevent trains colliding with the mirror.

Lift Gate

How about a lift gate raised by a micro servo controlled by an arduino that takes as input train sensors using toroidal inductance coils around the track feeders for the track block leading to the hole? 

Lol.

Or you could just lay a scrap piece of 2x4 across the hole when not in use.

Yep, now the song is stuck in

...

You use the cutout that you removed to make a temporary plug when you're not running trains...affixed to a mirror on the train side so the wife doesn't notice the obvious plug...

There are a number of possible styles of doors.

Apparently from your description, the area on the other side of the hole is neither heated or cooled.  Without any barrier, heat will always travel to the cool side.  In summer time, the heat will leave the warm room to travel to the colder (train) room.  In winter when you are trying to heat your train room, heat will go to the cold room.  Therefore, you want to put some sort of door there to stop the heat transfer.  You might want to insulate the other room to control the temperature better.  

Whether you install some sort of power operated door, or a manually operated door depends on how you want to use the space.  If it will be a regular part of railroad operations with quite a bit of traffic, or if it will only be used once or twice in an operating session, will determine what sort of door you would want to install.

I'm working on it today !

After thinking about this, I think there might be some merit with adding a little insulation on the backside of this wall. I have even thought about drywalling it all. I'm not sure it is worth doing if I am not heating or cooling the unfinished area? The builder finished out this room when this home was built in 2001. I didn't own it then, but I doubt there is any insulation behind the drywall, anywhere. That is a little disappointing, but oh well. My option would be to tear out the drywall and start again which is something I really do not want to do. 

The mirror idea was really cool and I am going to seriously try to take that into consideration. I think I will use standard studs and add more framing to the wall where the tracks will run. There are three levels and so, three separate holes in the wall. Completing the wall will give it some insulation properties and something sturdy for attachment of the benchwork.

As others have suggested, I will make some sort of door that will close over each level. I am thinking I could design this similar to a pet door. 

Pictures will be forthcoming to this blog thread.

If you want to see David Gilmore in a fantastic show, look up Pink Floyd in Pompeii. What a show that was ! 

Easier ways to insulate.

1. Drill holes in drywall, and use polyurethane expanding spray foam (think Mountains in Minutes) larger size spray cans for lower cost at big box home improvement stores.  Once foam expands, cut off or sand any foam that expands back out of the holes and fill with spackel.

2. Second option is to nail up furring strips over drywall, install 4x8 sheets of expanded foam insulation.  You could even use the white "bead board" type foam, then cover the foam with paneling.  No need to remove existing drywall, unless losing an inch or less of the room on all 4 walls is a big issue. 

Building...

A building with a working roll up door at the tracks would be a great way to hide that hole. 

Somebody made a working roll up door on here. 

Randy

So, Here is what I came up with

I do like the idea of a roll up door and I have been thinking A LOT about the real estate lost to modeling full buildings that do nothing but sit there. There has to be a way to do something with some of those. 

I thought about a pet door and some of those are "stupid expensive". If I'm just gonna blow money, it's gonna be on my hobby but not in that way. 

Then I remembered I had a dado set for my saw. As I thought about it, I thought that I could make a door out of framing lumber that could be insulated with both spray foam and fiberglass batt and then be dry walled. 

The first thing I had to do was cut the Dados. The Midwest Products Plywood door is 3/16". The Dados are just slightly larger to allow the door to slide freely. My door is 6" wide to accommodate a double track mainline and about 5 1/2" tall. The 2x4 framing lumber is twice as tall as the door so it can be completely raised above the track during an operation session.  

The door opens fully to ensure there is plenty of clearance

When closed, the door which is taller than the opening closes the hole in the wall completely which will help control air exchange and any humidity issues. 

It is not complete at this point. I will fill the cavities on either side with spray foam and fiberglass batt or some of my XPS foam board. The entire area except for the actual door / tunnel area which will need to be left as is for access and proper operation. 

Here is an overview shot of the door and it's relation to the overall wall structure. 

There is more to come. I got several pieces of the Midwest Products basswood. I will be working on the tracks that go through the wall. They will be made from Atlas Code 100 and the basswood will be built up to make a level surface for the door to close on. It will help serve as a transition between the Code 83 on the visible portion of the layout and the much more durable Code 100 used in the hidden areas. 

Looks meece-proof to

Looks meece-proof to me. 

Yeah you could build up the sides of the roadbed and between the tracks to provide a level surface. I was thinking the other way around, just carve the bottom of the door to fit down over the rails and roadbed. Dunno what's actually easier.

As for insulation behind existing drywall, besides the expanding foam stuff mentioned above another alternative I think is they have companies that will come in and blow insulation in through holes cut near the top of the wall (between every pair of studs). They patch the drywall for you afterwards of course. The same companies will blow that stuff all over the floor of an attic. 

The expanding foam stuff I don't know about. I'd want to know what chemicals it continued to out-gas after it had set up. Who knows, the blown insulation might not be all that healthy for you either, although both would be trapped behind the walls. 

Both expanding foam and blown in insulation are hazardous.

That is why you cover them with drywall or paneling so that you don't get off gasses and slows down fire going through the wall.  The only thing I know of that is not hazardous is the blown in cellulose, but that only works on a horizontal surface like an attic between joists.  To put it in a wall is like stacking sand.

insulation issues

I don't think I am too worried about insulating it. I have owned it for 6 years and haven't touched it. Part of me wants to make sure there is not any extra space for a helix or other track structure. It's always hard to see behind walls. Could you imagine finding another 10x10 space that was not finished someplace? Actually, I will find out soon enough. I am going to get down in the crawl and run network cable, phone lines (yes, I still have those wired phones), surround sound and cable lines so I have options when I redecorate or rearrange a room.

For now, I will just be happy with the continued progress on my doors. They have already turned out much better than I had expected. I will post more pictures as I get more done.  

Sliding door.

  I think the vertical slide door will work well. I'd build the roadbed/between the rails ballast  up level with the rail tops where it goes thru the wall then add some foam weather stripping to the bottom of the door so it seals the bottom better.   Mice can get in places you wouldn't expect. I've had them get into the heater/AC ducts on my pick up and build a nest on the intake manifold of my wife's Honda.......DaveB

Ooooh, I hate mice !!!

Here is the first hole I cut on the finished side. I have to do a little drywall repair before I hide the hole with scenery elements.  I think this hole will be concealed by piping and conveyors from the cement plant. 

I hate mice and have not had any, that I know of, for several years. I hate to poison them. It hurts the birds of prey. At the end of the day, they spread disease and due to their neurogenic bladder and other nasty bathroom habits. It just makes me shudder.  About a year ago, I bought the coolest little brick dealer diorama at a show. I realized pretty quickly that some of the loose bricks were really mouse turds... Oh, I just about vomited. That's just damned nasty. I can't imagine what kind of layout it came from. 

I decided to just lay the track without roadbed right in this special area. This way, I can glue the wood together. I just laid a piece of my Code83 with concrete ties across the opening. When I do it for real, I will transition to Atlas Code 100. 

As an Aside.

Depending on accessibility, could have a building flat (with attached bumper) fitted over tracks and held with magnets for when opening is not being used. Remove for operation.

More pictures

I used one of those old levels with the gradient attachment. I was at 2.5% which is more than I wanted, but I think it will be okay. Then I downloaded an app for my phone and it reads anywhere between 1.5% and 3.0%. It made me sad to know I was as steep as 3%. After thinking about it, I am okay with it. What it is going to do is add operational challenge. I guess we will see...

I curved the tracks as they went through the wall. That was a necessary thing due to not knowing exactly how I would build this thing. You can see the base layer of the basswood is made up of little pieces to make curving it easier. The top layer needs to be curved with a bandsaw or something, Even though I have  ton of tools, that is one thing I don't have. 

One more layer of basswood will finish the seal for the door. I glued a piece of basswood to the top of the door and now I have a handle to easily raise and lower the door.