I am visiting the Detroit area this weekend and am wondering if there are any excursion trains that run on a regular basis. I am not afraid to drive an hour or so.
Greenfield Village in Dearborn operates daily trips around the park. Although not a mainline run, steam is steam and they have some very interesting (and old) equipment. There is the Little River RR in Clearwater, MI, with trips that vary between 45 minutes and 1 1/2hrs in duration.
There is also the Michigan Transit Museum:
http://www.michigantransitmuseum.org/