Paul is a deeply troubled young man, hopelessly at odds with everyone and everything around him. He longs to escape from his relatively humble background and emulate the opulent lifestyles of the Pittsburgh captains of industry he so greatly admires. But instead of working hard to try and improve his lot in life, Paul retreats into a fantasy world, in which a superficial attachment to high art brings him closer—or so he thinks—to the charmed circle of the gilded elite. As he lacks the discipline or the application to make his dreams come true, Paul resorts to theft, stealing over $1,000 from his employers. With his ill-gotten gains, Paul takes off for New York, where he briefly lives out his fantasy as a wealthy young man-about-town.

But the money soon runs out, and Paul's plunged into despair. Aware that his crime has been discovered, he resolves to commit suicide. As we come to the end of the story, Paul's standing by a railroad track, ready to jump in the path of an oncoming locomotive. As soon as he makes the fatal jump, he immediately regrets his actions, but it's too late. As the train smashes into his body, Paul's "picture-making mechanism"—that is, his over-active imagination—is destroyed. Now, at the hour of his death, he's finally been deposited back into the real world he fought so hard to escape through his fantasies of wealth and social status.

Then, because the picture-making mechanism was crushed, the disturbing visions flashed into black, and Paul dropped back into the immense design of things.

The above is that last sentence you mentioned.  In this story, Paul is wrapped upon in his visions of a better life.  He's trapped by the American dream, which has him convinced he can only lead a good and happy life if he is leading a socially successful and glamorous life.  This dream can be compared to the moving pictures on a movie screen.  The flash in front of his eyes, constantly, distracting Paul and distorting his reality.  As he gets lost in the dream, he fails to understand that he is still just the same young man he always was.  He begins to believe he has transcended into a man and a lifestyle of more notoriety. 

However, this last sentence ends Paul's dream, and shows readers that it is a false and a destructive one.  Paul "falls back into the design of things" because his reality was false.  He was not meant to be of that higher social class, and it is not his world.  He does not stand out; he is another person on another trip through life, and must sink back into the reality of his existence.