My old stomping grounds. Tom's on the right track. (Sorry) For some outstanding railfanning, head to downtown St. Paul, right down next to the Mississippi River, on Shepard Road. I've included a few maps as guides (roughly photographed, but you can check Google Maps for better images. Drive all along Shepard Road heading east, and it will turn into Warner Road, and then peel off onto Childs Road. Pretty soon you'll bump into a huge Canadian Pacific yard, the old Pig's Eye yard, below Dayton's Bluff. There's a UP yard next to it as well, but there's plenty of action to occupy you at the CP yard alone in the time you have available.
There are maintenance buildings, even a operational turntable for turning the diesels, and lots of engines and vast amounts of rolling stock. The yard goes on and on to the east. I've driven around the yard quite a number of times over the years, and never been stopped. Just use common sense and stay a reasonable distance from the tracks. When I first visited there, there were Soo Line, Wisconsin Central and a few Milwaukee Road engines on the property, plus a number of foreigners. Today you'll see mostly CP in a variety of schemes, but there will be other interesting engines hanging around as well. Google the yard - there are plenty of pictures and other information.
There's also a fantastic museum just north of downtown, the Jackson Street Roundhouse, but you'll probably need to save that for another time.
