Well, for starters, there is no more Homestead Works. It is totally gone and there is a really upscale shopping district called The Waterfront there in its place. The only thing left standing are 6 smoke stacks and a gantry crane. There are still a few mills still operating. The Clairton Coke works is on Route 837 in Clairton. You'll catch some NS action and heavy Union RR coke trains that make the climb to Dravosburg about every 3-4 hours. You can hear those MP-1500s scream usually around the 45 minute mark. This is very impressive to watch. You can walk faster than these 5-6 MP's.The Edgar Thompson works is still fully operational and you can get some great views from along the roadside on Route 837 in West Mifflin near Kennywood Park. There you can see NS, CSX, AmTrack & Union, along with ET's own fleet.
There are 3 abondoned sights on 837 also. Between Clairton and West Mifflin is what's left of the USS Duquesne Works and the Dorothy 6 furnace, across the river are some buildings from the National Tube Works in McKeesport, and just past Kennywood across the river from Homestead are 2 large gantry cranes and a battery of furnaces.
Actually for train spotting The Waterfront area is great. There is a spot on Amity Street (one of the entrances to The Waterfront) at the old PRR Station that you can see both NS and CSX. They pass within about 30-40 yards of each other and you'll get coal drags and a lot of intermodal action on both.
I hope this helps and have a great time in Pittsburgh. Go to Primante Brothers for a sandwich...you wont be disappointed!!!!!