Another place to try that is much closer to Ann Arbor is Romulus. In fact if you are flying into DTW you will land in Romulus. The NS line (former Wabash) that runs from Detroit to St. Louis crosses the CSX (former C&O) Toledo-Plymouth line at grade in Romulus. You can park in a neighborhood that is near the diamond and have good viewing. Both lines can be moderately busy, particularly in the morning. CP also has trackage rights on the NS line, and if you are lucky you might catch one of the NS RoadRailer trains. Milan is another possibility. Just south of Ann Arbor, in Milan the aforementioned former Wabash track crosses the Ann Arbor at grade.
The AA bridge crosses both the NS and the Huron River not far from the Amtrak station. The Art Train used to be based there, too but it was converted to Art Truck a few years ago
I'm sorry but I don't know the times for the couple of NS freights that run on that line. With the exception of a short stretch between Detroit and Dearborn, that entire line is single track, which means the two daily NS freights have to time it so as to stay out of the way of the six Amtrak trains. You might ask the Amtrak agent in Ann Arbor if he/she has a sense as to when the couple of NS freights usually run.
If you get a chance to visit neighboring Ypsilanti, I highly recommend eating at the Sidetrack. This bar/grille is a long-time institution and a favorite for families and college students, too. And you can't beat the location, hard by the NS line that runs through Ann Arbor. The area of Ypsilanti where the Sidetrack is located is called Depot Town.
Let me know if you have additional questions!