[cardiffspaceman]

I think the basis could be that the newer trains were too light to stay sunk, being made of a material that is lighter yet stronger. Also it so happens that construction details matter. The article cites corrugation, which allows panels to be lighter yet just as strong. The article also cites spot welding. You can weld together steel panels in many ways, and the article doesn't say how the Redbird cars were constructed, except there's an implication that there's no spot welding. The Titanic was made of "iron" as the dramatized architect said in the movie, but he seems to speak poetically [1][2]. The ship hasn't quite dissolved like an aspirin.

[1] https://www.moviequotedb.com/movies/titanic/quote_23821.html [2] "The 2,000 hull plates were single pieces of rolled steel plate" https://en.wikipedia.org/wiki/Titanic#Building_and_preparing...