The northwest building has a couple of interesting spots (1-3).
1. Go to the warehouse section of that building. Go up the stairs and climb the ladder. Turn around and you will see a pipe that runs across the room. Get on the yellow railing and jump and squat to land on the pipe. This overlooks the warehouse area and you can watch out of the northern window.
2. Go to the stairs below the roof that people snipe off of. Break the window below that sniping roof. Climb out onto an inches-wide ledge and head left around the corner (on that ledge). You will end up in a dark corner that looks over the blue fence and you can see most of the middle section of the map.
3. Jump off of that ledge and get between the fence and the southern side of that same building. Head towards the south-eastern edge of that same building until you get below the second floor window of the warehouse section. Look for the lone pallet on the ground and get on it. Jump to the box by the fence, face the building and jump onto a box there (next to barrels), turn around and jump onto the barrels by the fence, turn and jump onto the barrels by the building (you are basically jumping back and forth, getting higher each time) and face the window upstairs. Jump up and you may have to hit A again to get your guy to climb into the window (this is a moderately difficult jump).
4. Head to the transformers section in the Southwest. Come out of the tranformers heading north. Look to your right at some boxes and containers next to the transformers, near the middle building. Jump on the left boxes, climb up left side; this overlooks much of the northern half of the map. Turn around, jump onto the middle box, turn left and jump onto the other side of the middle container; This overlooks much of the southern side of the map. From here, look at the highest stack of containers, jump at the corner there by the middle container. You should climb an invisible ledge that puts you on top of the middle container that you have jumped around. This gives a wide view of the map.
Now, from here you can jump on top of the tall blue fence that surrounds the tranformers. Head south (on the fence) and jump the opening. head west (on the fence) until you get to the western edge. Here you can fall onto 5 large snow-covered containers and nuke boost or whatever.
Extra: From the high sniping roof of the northwestern building, you can jump to a street light and down to the fence and head over to the southeastern window described in 3, above. Also, you can jump onto the green jeep (range rover?) and onto a container and then get over the fence by that same window.
I hope this was all clear enough.
All the other roofs/train cars spots should be obvious.
[more cheats for Call of Duty: Modern Warfare 2 here]