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Someone on another site was asking who made those vacu-formed, or other method style of premade layouts? I remember seeing an American Flyer one about 1964, and I believe Roco and perhaps Fleischmann made some. they had the mountain, perhaps a tunnel and might have been a twice around design with 1-2 sidings. I think the scenery was done and it had a few structures.
Here's a scan from a catalog from about 1959 or 1960. The HO layout with the town site looks to be designed specifically for old Plasticville structures. These appear to be a bit different from the American Flyer S scale "All Aboard" S scale system of interlocking squares with track and scenery attached.
Premade HO scale layouts by Noch show up on e-bay from time to time. Noch made several different preformed layouts. Those were from the 70's though I believe they still make them. Marklin made some as well. In the 70's Life-Like made preformed layouts molded out of styrofoam. More recently Terrain for Trains currently makes an HO scale vacu-formed layout. Most of these required laying your own track and adding your own structures.
I had an American Flyer S-scale All-Aboard (as in the link in the prior post) with six pieces that could be arranged in a few different ways. It was a simple flat oval. I did not realize it had different possible pieces. Unfortunately that was during the great decline of AF. I liked the modules but the trains and track were just junk.
I do remember the stores carrying this sort of layout. Seems they were plentiful. For some reason I'm thinking both MARX and AHM made one.