NS has 8 daily trains crossing the Genesee Arch Bridge these days- 22K, 23K, 28N, 287, 309, 310, 36T and 37T. To the best of my knowledge, 309 goes on duty in Binghamton around 11 AM these days and 36T goes on duty in Buffalo during the early afternoon. NS 28N tends to be a mid-day train. With that being said, the times these trains cross the Genesee Arch Bridge can vary depending on the work they must do in various yards, as well as waiting on sidings for traffic moving in the opposite direction. Some of these trains can have extra sections- most notably NS 310 (W10) and NS 23K (I3K). IMO, the best thing to do is stay near the bridge for a few hours with a scanner if you want a decent shot at catching a train.
"It is highly likely that the 1990s were an overrated decade."