Building a 10x25 for the Lionels........

Put as many main lines as you can squeeze in. I offer this simply as I like to see as many sets running around as I can shoehorn in on the floor or on a platform as I can. I’ll leave the structure particulars to the experienced experts on this site.

BMiller

Start by pricing them at HD to have that data.  Find out if you can obtain any used or free or on sale, after all, this is just a foundation for the layout and will not be seen.  Then, YOU have to do studying and make a chart of advantaged and disadvantages of each including your experiences as to working with each.  Then, make an informed decision on which way to go.  Do not over think this as other things are much more important like track plan, operation details wanted, control system, train gear you want to operate, etc.

I know many others will have lots of suggestions as to their favorites which should help learn about the choices.

I used some free 3/4 inch thick Bagasse board, which a particle board made from sugar cane residue in Jamaica, as I knew the plant manager who offered it to me.  For the expansion in the states, I used cheap grade HD 1/2 inch plywood that was noisier and I installed truck inner tube rubber pads under the ties cut down the noise.

Charlie

You’re going to want to consider having to access the middle of a huge table. If that’s the case, make it’s sturdy, use plywood, that way you’ll be able to crawl on top of it to access the center if need be. My 12‘ x 8‘ table is 2 x 4 construction with 1/2} plywood and 4‘ x 8‘ sheets of sound deadener board I got from Lowe’s hardware.

In that space you could do a folded dog bone layout which would visually and operationally be more interesting, not to mention allow better access around the layout.

What type of scenery do you want to do? Others who have done this can chime in but one method is to have a layer or two of Styrofoam so that you can carve scenic venues such as rivers into the layout. You can also consider cutting out the plywood top to match the curve of the track and leaving the rest of the table open framework to accommodate any rivers or valleys you might want to incorporate.

All you need is 1/2" plywood and homasote for the top. That will give good sound deadening, and it gets even better if you use a quality foam road bed. Those other materials you mention can be used for scenery.

A lot of people over-build their benchwork, the primary culprits being the use of 3/4" plywood and a lot of 2x4 lumber - these things are rarely necessary.  Look up L-girder benchwork, originally designed by Linn Westcott of Model Railroader - it may look fragile, but it's fully sufficient for toy/model trains.

With a 10x25 layout, be sure to include several interior openings (or pop-out scenery panels) so that you never need to reach more than about 2 feet to get to any part of the layout.  Although some people walk on their layouts, I'd advise against it - trying to work on something while keeping one's feet in specific locations is like playing Twister - and even if you're agile enough now, you'll get older.

Lots of threads on the forum regarding the use of different types of plywood, homasote, cork roadbed, and foam.   I'd suggest contacting some nearby folks and asking to see what they've done on their layouts - pick their brains about lessons learned.

My main layout is 21' x 7.5'.   I used 1" x 4" joist for the frame with 1/2" ply topped with Homasote and Roadbed.  I weigh 240lbs and can/do walk, crawl and lay on it.

Sound deadening, about 65db is conversation level...

BMiller, congratulations on being able to create an empire of those dimensions. My layout, still under development, will have one inch foam on top of the plywood. This is for purposes of scenery. My intention is to carve out a ravine between my mainlines and to be able to recess structures and alter the terrain in other areas.