The description has it the other way. "The solution is that, even though the trains arrive with the same frequency, the downtown train departs just a few minut

The description has it the other way. "The solution is that, even though the trains arrive with the same frequency, the downtown train departs just a few minutes after the uptown train has departed. Because of this, there is only a narrow time window in which the man is able to get on the uptown train." s/after/before/ to fix the explanation.