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I think recommendations may depend on what your approach is to building the model.? Plastic/brass/3D print?? Scratchbuild/kitbash?
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My general approach, if rtr or side kits are not available, is to 'cut and paste' using parts of various Micro-trains (or Rivarossi in the old days) heavyweights to get approximations of the window arrangement.? I don't see why that wouldn't work in roughing out the enclosed obs deck and for the windows at the end of the car.? However, the end result is usually an approximation of the prototype, not a precise model.? If using this approach, note that the MTL obs has 4 close side windows, which are also one of the 'spotting features' of the plan 3521 cars.? Before I found a brass 3521 (the version used by ATSF- see below), I was considering using parts of a 10-1-2 and the MTL obs and thought I would get close.
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Another thing that might be worth looking into- Pecos River Brass did earlier versions of the 3521- which had the lounge end enclosed with a vestibule (that is, it had doors at that end, not a solarium, although they did have a pair of end windows).? They also did an open platform version. As far as I know, they didn't do an enclosed solarium version, but the 3521 models do show up on ebay or used brass vendors now and then.? Reworking the end into a solarium would be old-school brass fabrication, and out of my league.
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Tom D
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John Perkowski