If you have six hours, you can cover a lot of ground, especially if you don't mind a few taxi rides. My favorite spot in the city is the north throat tracks to CUS, where they cross Canal street. You can watch Union Station, Northwestern (Ogilvie) Station, and Elevated traffic. Other good spots include Roosevelt road, which spans approach tracks to CUS-South, LaSalle Street, and the Electric terminal under MIllenium Park. Since its the weekend, print up some timetables because Metra traffic is sparse on weekends. Don't bother going into LaSalle station, as the only thing there on weekends is the protect engine and maybe one train. There will be two trains each parked in the upper and lower concourse at Millenium Park. Northwestern and Union station are always a grab bag for action. Don't be a blatant railfan, but I take pics there some days.
If you get a locker, park your gear there. #6 and #448 both use the south concourse, so use a locker in that waiting room/hellhole. Walk through the station and onto the north concourse platforms, then up the steps to the street at the far north end of the platforms. It's a five minute walk to the Canal crossing. If you like airliners, stop at the Boeing store in Boeing HQ - it's on the way. After you've spent time there, either take a 20 minute walk or hail a cab, depending on your preference, to Canal/Roosevelt. If you're hungry, Manny's deli is right around the corner and makes amazing sandwiches. Cash only. Start to walk east, toward the lake. You will first cross the CUS yards, then the river, then the LaSalle approach, then under the L, then reach Metra/South Shore electrified trackage on the lakefront. It's a 10-15 minute walk, but stop for pics wherever you like. Nobody has ever hassled me on Roosevelt. Once you hit the electrified trackage, walk north or hop a ride on Metra to Millenium Park. They're railfan friendly in the upstairs (South Shore) concourse. Not so much in the downstairs (Metra Electric) concourse. After you are done there, walk or cab back to Union Station - 10-15 minute walk.
I walk this all the time but I don't know your preferences. Cabs are cheap and easy to find. You could also board a brown-line L train and ride around the loop, but you still have 5 minutes of walking - Union Station is across the river from Quincy L station.
The new Acela: It's not Aveliable.
