Tom writes: This is a Railroad Bridge shown in the 1929 #10 manual.
It is in the section for models to be made with the B accessory set or #10 set with additional parts. The only major difference with my version is that the horizontal EX girders shown in the manual are not used. My reason was because I did not have enough EXs and they seem to be only aesthetic, not structural. They appear to be a veneer on top of the structure. The bridge as I made it is very stout due to all the trussed square girders and would support a substantial weight, though I have not tested its limit. At five feet long it uses a lot of parts: 56 Bs, 136 Cs, 28 EXs, 10 EYs, 92 Is (or substitute Hs for some Is), 10 MN plates, 8 BE, 28 AH, and many CHs. I used almost all the nuts I have and needed to buy more long 8-32 screws in 3/4 and 7/8 lengths.