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I can't comment on models -- no experience there. Every photograph I've seen of freight car manufacture of the wood-frame era shows the car being assembled right-side-up. There is very little underneath the car--just the brake equipment (the truss rods are part of the structure)--and while overhead work is inefficient and slow, the amount of it was apparently deemed less expensive than to purchase cranes or fixtures that could rotate the frame after all the parts were added. In this era labor was cheap, machines expensive. The photos I've seen of car assembly using cast-steel underframes appear to follow the same practice. When freight cars moved into the fabricated steel underframes era, however, upside-down assembly took on a new aspect: welding overhead is something to be avoided whenever possible because the weld quality is very difficult to maintain. But most of these frames can be assembled by reaching through and there's no advantage in an upside-down frame, assuming one is then applying a wood floor. I've seen photos of car assembly in the 1940s and 1950s showing frames being assembled both upside down and right side up; the general practice in either case seems to be to get the frame onto trucks as soon as possible in order to move the car down the assembly line with little effort. Modern steel cars in every case I've seen use a rotating fixture, not so much for the addition of brake gear--though that is made more efficient--but to lower the cost of welding and to obtain consistent high-quality welds. Transit cars are assembled right side up, even though there's a great deal of equipment, piping, and wiring beneath them. We toured the Siemens shop in South Sacramento last year and observed this. I asked about it and the shift foreman conceded it would be more efficient to do the underfloor work with the car flipped over, but then they would have to build fixtures for this, and given the small production runs and variations in car lengths, etc., they had calculated they couldn't offset the cost of the fixtures with the labor savings. Locomotive manufacturers, as you note, assemble the frames upside down. As you observed, locomotives have considerable amounts of underfloor work, but unlike transit vehicles production runs are much larger. Perhaps more important is that the underframe components of a locomotive are heavy, bulky, and unwieldy, whereas practically everything beneath a freight car or transit vehicle (except in the case of freight cars the air brake valve and main reservoir) is fairly lightweight.
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