I believe that Lionel, AF, Marx and other toy factories employed many day-workers also many women, some experienced, some not so much. Tab and slot construction is kind of a no-brainer, as long as you know which parts to put together. T and S assemblies were not made to be taken-apart for service or future restoration. T&S was a one-time fastening method, that streamlined assembly, much faster and more economical than rivet or bolt/nut assembly.
I am sure that future restoration or rewiring due to insulation loss was not foreseen, but Lionel must have realized at some point the whistles in their Junior /027 and 263W type Vanderbilt tenders would require cleaning and lube at some point. 1689T/W tenders usually will come apart and re-assemble without tab-loss as long as you are careful. However, the tabs underside a 263W or 2263W type tender are difficult NOT to break, as they are a thicker/harder metal. Poor-planning for what was part of "top of the line" sets.
When we made Marx Trains in the 1990's, most of our car and loco assemblies were T&S like the vintage originals. We were lucky to have some of the original plans from Marx in Miami. The litho material we used for most cars was actually thinner than vintage Marx .010. Tabs were very easy to bend and re-bend due to this thin material.
(I would have to ask Sheldon about the physics behind why thinner material is more elastic than thicker material...LOL)
The cow-catchers/pilots on some of our locos were much thicker and some were chrome/nickel plated. The tabs that secured the pilot to the bottom edge of the sideboard were hard to bend, often resulting in a slip that scratched the sideboard. Un-bending these tabs was even more-difficult due to the hard-brittle nature of this material.
