Well, you can really make a good walking tour if you head south on Canal out of Union Station. The first stop is about two blocks past I 290, I think its Polk street. It is just past the 'new' US Post Office. Make a left and head towards the east. You can walk out on the bridge right over the south tracks heading into CUS. It is a dead end bridge with maned check points for the Post Office, (the check points are on the opposite side so they don't affect you going out there). I've walked out there and taken pictures many times and nobody has ever bothered me. Head back out to Canal and continue south. Next stop is the previously posted Roosevelt Rd. Its a pretty big bridge that spans the yards of Amtrak and Metra. I have been questioned by police up there about what I was doing. I told the officer that I was a rail-fan and I was just taking pictures. He seemed satisfied with that and went on his way. The whole area has been newly updated so there actually are other little points to stop and look out trackside between Polk and Roosevelt. There's a nice little diner to take a break in on the corner of Roosevelt and Canal. The food is decent and the place has been crowded every time I've been by there. Continue South on Canal down under the old St Charles Airline where the City of NOL, Saluki and the Illini come across and shove into CUS. Then another short bridge takes you right over the south end of the Metra yards and the Wye where Amtrak trains are turned for servicing. That bridge used to be wide-open but the city ruined it when it was redone. Now its hard to see past all the decorative cement, let alone take pictures. You can head north about half a block off of this bridge and get down trackside for a few decent pics. Next head down on Canal to 18th street this old bridge takes you right over Amtrak's south end and near the Diesel shop. From this bridge you can get down trackside, there's a nice stairway that leads down to Lumber street where again you'll be trackside for all the trains comming in from the south. That's about as far south as you need to go. There are a few other good spots right around there but I'm not real familiar with that neighborhood. All told its probably about a mile down and a mile back. Its tough to catch a taxi coming north on Canal so when you leave CUS program a number off the cabs parked out front into your cell. I think it costs around $5-6.00 but I usually walk it all.
All in all its a safe walk, I've never had any trouble so have fun and I'll see you out there!