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.
"Then, because the picture making mechanism was crushed, and disturbing visions flashed into black, Paul dropped back into the immense design of things."
I think the disturbing visions were the scenes of the Adriatic water and the yellow of the Algerian sands, both the images he realizes he will never see now that he has jumped in front of the train. These images are disturbing because they will haunt him even in death because he will never see them.
When the disturbing visions flash to black, this is a reference to his life on Cordelia street, where there are no cut flowers and no colour. Now, he is dropped back into the colourless and immense design of things which he had tried to escape by killing himself.
Or, : ) he could be dropped back into the immense design of things by realizing he will never get to see the Adriatic water or the Algerian sands and therefor he is exactly like the people who he despises. He sees his peers, teachers and father as people who just live life by doing what they are supposed to and now he has ended his life before doing anything of significance and is just the same as the others.