North by Northwest's most celebrated—and sneakiest—symbol appears at the very end of the film, when the train becomes an enormous phallic symbol, as Hitch himself declared (source). The train entering the tunnel is meant to remind viewers of sex: specifically, a you-know-what entering a you-know-what.

In case we don't get it, the scene is superimposed on the words "The End."

But there's a less obvious—and much less larger-than-life—phallic symbol in the film as well: the small razor that Thornhill uses to shave after he finds it in Eve's bag. For the critic Raymond Bellour, this mini-razor's a phallic symbol that hints at Thornhill's emasculation: early in the film, when he's still under his mother's influence, Thornhill's not fully a man (source).

You may roll your eyes at this Freudian interpretation, but it's worth thinking about. It was a way for Hitch to stick it to the production code guys, since they'd insisted on some dialogue and plot changes to satisfy the strict Hays Code.

Come on—no one tells Alfred Hitchcock what to do and gets away with it.