Best way to expand my 4x8 table

I vote four legs, like your last table. Then screw the two tables together.

Not sure you need a 2x4 construction.  I used 1x4 construction on my 8 foot by 20 foot layout and it is plenty strong and I can crawl on top of it if need be.

As for my multiple table sections, I suggest adding legs only to the freestanding end of the table and attaching it to the existing layout.  I just added wheels to my layout and the 1x4 construction combined with having only 1 set of legs (every 4 feet at the joint) along the joining of two tables, is plenty strong and the table moves with ease.

Here is a view of the underside.  There are L-shaped legs, constructed of 1x4s every 4 feet from front to back and from side to side.  The multiple sections are screwed together with only 3 legs, spaced every 4 feet, along each table section joint.  When adding the wheels, I added 1x4 bracing from the front to the back of the layout and plywood bracing/shelves from side to side.  This has created a very stiff and sturdy layout that can be pulled away from the wall to access the rear of the layout (and breaker box) as needed.

Save the lumber, use 2 legs and screw the other end into the end of the existing table. Now if you are building an extension "bridge" between 2 tables, just hang it from the tables - no legs.  2x4s are overkill, but its your RR, you can do what you want. 

I am building a new version of my RR.  1x4 L-girders, 1x4 legs and 1/2 inch baltic birch type plywood. 240 pounds on the middle of an L girder 6 foot centers - deflects some but doesn't break. Next benchwork tables are going down to 1x3s.   

Build one more set of legs for the outside end of the extension. Bolt the new table to the existing. The 2x4 framing will handle the extra weight without a problem.

Have you given any thought to the legs being 'set back' four or five inches from the edge of the table?  John

Attach a 1x3 lip to the side of your table that gets the extension.  Build your extension and hang it on that lip, automatically levels the extension to the existing table.  Add 2 legs on the opposite end. Voila,  done!

I went with the 4x4 extension to my table using 2 legs and screwing it to the original table.  Worked great and saved me some expensive lumber at the time.  Only advantage I could see to 4 legs is that if you wanted to break the table up to move it for any reason that would be easier.

In addition to lengthening the table, you might want to consider widening it.

Even just a little to 5' wide will allow you to lay O-54 Fastrack curves which will, in turn, allow you to run all but the biggest O-72 equipment.

Thanks for the replies, everyone. Good suggestions. I certainly want larger diameter curves, but will probably be saving that for future layout building (something more permanent) after the basement gets finished in a few years. For now, I can run traditional and smaller Legacy equipment. The outer loop has that "compound" curve of around O44, which is helpful. I would be curious if an O54 diesel would run it since it is just a loop, no switches, etc...but would probably be too tight.

I'll probably go with a 4x4 extension, with two legs, and connecting it directly to the existing table for support as a few of you suggested. The 2x4 construction is a bit heavy, but it's my first benchwork build and I'm no carpenter, so it was easy to work with and I simply followed a video I found online for the original table.

Final alternative may also be another 4x8 table, into the L-shape configuration, to expand into one of @Ken-Oscale 8x12 L-shape layouts. A little larger, more track, etc... I'll have to decide if I want to take that on or stick with the 4x12 for now, which would likely be based on Ken's 4x10 layout, but probably just expanded a little to fill the 4x12.