Once you are on the dock, head up the steps to the first area and take cover. Wait for the patrol to pass by and they will form a pretty strait line. You can unload on them and kill most of them pretty quickly. Head up the right side and you'll approach the subway. Just clean up the locust on the way up to the platform. When you finally get rid of the last grub (all Theron actually), a train will arrive. Kill the passengers and there will be a troika at the back. Shoot the red tanks next to the troika to destroy it. Be sure to grab the collectable, on the far end of the outdoor section of the platform (as opposed to the indoor part where the train is).
*cutscene*
Head through the newly opened door and after a bit you'll be told to look for a silver back. You'll find it to the left. Hop in, go through the door and follow the grassy tunnel. You'll eventually get to a Reaver, and beyond that, a beach. Make your way along the beach taking the low or high road. Watch out for the boomer at the far end. Kill the boomer and when you are told about the mortars, turn left and head into the tunnel. In the tunnel, if you keep your eyes left, you'll find a gated area. You'll need to use the silverback to kick the gate down and then in the back corner of the room is a collectable. Follow the tunnel killing enemies as you go. You will come out of the tunnel and have a few more grubs to kill but when you come out of the tunnel, try and stay left and you'll move into some trenches.
In the trenches you'll encounter some grubs and a boomer. Just after the boomer will come a group of wretches. They'll be very close so remember that the silverback can melee. After the wretches you'll come to a dead end. Turn around and head back the way you came. As you come out of the trenches, a couple of boomers will open a door for you. Kill them and you'll come to another subway section. Wait for the train to go by, and then kill the Kantus if you can find him. Try to kill as much as you can with the Silverback. You'll have to hop out to open the gate on the tracks.
Walk up the tracks and a train will come toward you. You can try and keep the silverback but I ditched it here. Come up, out of the train tracks and you'll get attacked from the other side of the tracks. If you run up the stairs on this side of the tracks, it will bring you down on the other side of the tracks to flank them. Clear them out, including a couple of Boomers.
