Some flat bed semi trailers are designed the same way. It is interesting to watch a single heavy item placed on them, and see the "positive" hump flatten out. A lot of design went into this type of trailer. The hump has to be accounted for in load heights, otherwise serious incidences can occur....
As for the rail cars, I have heard this "story" many times, and have also seen model examples. I would swear that I have seen very good prototype examples of this, but I cant find any, or remember specific examples....
Casey