Andrew - what are you going to do about the upper level supports? Lionel used turned wooden columns for the upper level, which was made from plywood and thin trim stock.
The turned columns look suspiciously like large turned wooden commercial thread spools. Are you planning to turn your own columns? It is interesting to note that although the D-148 was a 1955 display layout, and the 110 graduated trestle set was a 1955 release, both the D-147 and D-148 did not use them. This can be explained by the fact that the display layouts were always designed in the late fall of the previous year and the prototypes made prior to Toy Fair, which happened early in February of the catalog year. So although the 110 trestles may have been available in 1955, they were not available in fall of 1954 when this layout was designed by Bill Bonanno and his team of Department 15. There were some changes from the catalog illustration to the production version of the 1955, most notably the two ended siding on the "front" board instead of the stub siding, and the "horseshoe" tunnel portals instead of the "arch" style, which were used in 1952-1954. Attached are some photos of an original restored D-148. These may be of some help to you in building the mountain. Please note that the "elevated" portals are extra tall and extend all the way to the plywood base. The upper level is the same 3/8" plywood used for the base that is cut out to follow the shape of the upper level track and is supported by 1" X 3" wood risers. The entire inside of the tunnel is tan, and there would be screen printings of the green areas around the base of the tunnel portals to locate the wooden "feet" of the portal, which are attached at a 90 degree angle to the wooden tunnel portal. The presence of grass on these "registers" prove that the mountains were added after the entire flat base was completed. Don't forget to add an additional small portal at the back of the mountain. Lionel always did this in order to provide access to the inside of the mountain for derailments, cleaning track etc.
