If the downtown train arrives and departs a few minutes after the uptown train arrives and departs, respectively, why would he take the downtown train more? It seems to me that if he missed both trains, then he would take the uptown train, as it would arrive earlier.
>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.
He should say it like you suggested though.
"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.