I used 7 ply birch 3/4" plywood. Never a problem.
Your spending thousands on your trains but you cheap out on wood, I don't get it.
Remember when some were a proponent of using lamp oil for smoke units, how has that worked out?
Maybe I am not understanding something here why even use plywood? why cant you directly adhere 2 layers of 1" Rigid Insulation (pink or blue) directly to the framing, stagger the joints and change the orientation of the second layer? to me the plywood seems like overkill
In the time this post has been running, you could have stopped by any new construction jobsite and scrounged enough 3/4" underlayment scraps to build your layout. It doesn't get any cheaper than free. And you'll never get anything built by asking the same very basic question a different way every other day.
I used 7 ply birch 3/4" plywood. Never a problem.
Your spending thousands on your trains but you cheap out on wood, I don't get it.
Remember when some were a proponent of using lamp oil for smoke units, how has that worked out?
Why is it over engineered? It never has warped, it is very stable. I also put 2" of pink foam on top. The base is also made of 2x4's that are bolted together, the plywood is then screwed to the base using 6" screws.
I believe in doing things right the first time. I have spent many thousands of dollars on my trains and I was determined I would build the layout correctly.
If you want to do it with a cheap grade of wood then please be my guest.
Yeah what he said! 
I don't build a layout worried about someone else buying the house down the road. Has nothing to do with over thinking it. Not everyone builds portable/modular, around the room layout. Don't knock it because it's not the way YOU do it!
A door skin braced with a few 1x3's is not going to support this:
quote:Maybe I am not understanding something here why even use plywood? why cant you directly adhere 2 layers of 1" Rigid Insulation (pink or blue) directly to the framing, stagger the joints and change the orientation of the second layer?
My primary reason: the rigid insulation I've seen does not hold screws worth a darn.
I've helped my older son build several HO and N gauge layouts with rigid insulation. On the positive side, it is very light, and can be sculped.
Woodland Scenics Foam glue, pink or blue insulation sheets, and black plastic modular shelves as benchwork, is all I use.
No heavy wood, no fiddle faddle with nails and screws, saws etc.
The glue is tough as nails, and forgiving if you make a mistake.
No need for plywood, but I have used the above mentioned glue to adhere insulation sheets to foam.
If you decide not to use plywood, then you have to use thicker foam or multi sheets for thickness to stay rigid.
Just another case of different people with different ideas about what is "the right way".
Some people are always talking about how they walk on the layout all the time and they need it to hold 250+ lbs. I guess they should build it like an outdoor deck.
Others have well-placed access panels or plan the bench work for ease of access. These people only need benchwork sturdy enough to provide a rigid platform for the trains.
Others look to the future and the probability of moving it all. These people plan light-weight modules small enough for easy moving. Good design and bracing take the place of heavy materials for these builders.
The moral - build your benchwork to meet your needs and with materials that fit your budget- not what others say is right. There isn't just one "right way". 
Jim
Just another case of different people with different ideas about what is "the right way".
Some people are always talking about how they walk on the layout all the time and they need it to hold 250+ lbs. I guess they should build it like an outdoor deck.
Others have well-placed access panels or plan the bench work for ease of access. These people only need benchwork sturdy enough to provide a rigid platform for the trains.
Others look to the future and the probability of moving it all. These people plan light-weight modules small enough for easy moving. Good design and bracing take the place of heavy materials for these builders.
The moral - build your benchwork to meet your needs and with materials that fit your budget- not what others say is right.
Jim
I can barely bend over, I cannot imagine walking on any layout.
Would like a topside creeper though.
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