Along with that, alloys and various improvements allowed for less labor intensive means of creating a supporting structure that was "smaller", stronger, & better use of resources and more flexible.
The straight channel sill was found to be sufficiently strong and reduced rivets, time and steel. When weldments came on line that further reduced time and materials. A jig ended up doing the work (or allowing far fewer to work) on the whole structure at the same time.
The same thing happened for passenger cars, but it took a lot longer (for a variety of reasons). the sill does more than "just" impart structure to the frame, it can also lower CoG and provide additional stiffness in specific planes (that otw might fail in certain circumstances). A Standard Pullman was called a battleship for the keel (sill) and the heavy riveted side frame and skin that mimic the (essentially, pre-Dreadnought) Battleship construction.
People tend to forget that huge swaths of industry and society have seen a sudden and virtually entire loss of functions. The Milk industry (& usually associated ice industry) is one large example (used to be 10 ice houses & 4 milk concentrators within 5 miles of here) . But at the same time, the consequences of specific construction methods - of buildings, roads, autos, and freight cars - all changed heavily in the 1910-1940 era. A great many methods WERE regulated out of existence, a great many were simply passed by events.
For instance....driving through beautiful downtown Elgin today, most of the large brick buildings making up a semi-industrial corridor are gone in the last 20 years - and in the resulting open space, you can see that now passed over tech, the pay phone (complete with sign pointing at it) now used mangily by dealers of all sorts. But no one regulated land lines (esp. pay variants) out of existence, some few remain, but the rest have simply outlived their usefulness (and may yet be regulated out of existence due to their use in crimes).
At 01:56 PM 9/28/2019, Dennis Storzek wrote:
And still in use long after the cut-off date of this discussion group. TTX 89' flatcars have fishbelly underframes. The reason we don't see any on house cars built after 1930 or so is designers finally realized the design was overkill; it added weight whereas a straight sill was sufficiently strong.Bob Webber