The next section is short and you just have to shoot down a couple of barges with the turrets. Aim high and it shouldn't be a problem. Once you've done that, you'll drop down to the ground for a rescue. If you turn around and follow the path, you'll find a full ammo crate if you need it. From where you dropped, you'll be attacked from directly ahead and from above the gate on the left. Eventually you'll be attacked by a Reaver on the building ahead and left. Kill the Theron directly in front of you. They should drop torque bows which will be helpful for killing the Reaver. Take out the Reaver and if that doesn't trigger the cutscene, sometimes there's a Theron hiding in the building up the stairs.
Grab ammo and whatever weapons you need. You can fill up on ammo, grab grenades and there's your choice of Retros and Longshots on one wall with hammerburst, torque bow and Boomshot on the other. When you're ready, grab the one shot leaning on the crates and head out. The big gate that was previously closed is now open. Try and save the OneShot because it should actually take out the reavers in one Active Reloaded shot (if not a regular). There should be 5 reavers over the course of the encounter in the next street so make sure you save enough for them. Once you clear the area, head to the left end of the street (by the wrecked truck) and there should be a collectable on the ground. Head back up the street and on the left side should be stairs. Go up the stairs and take a right. Head into the one room building directly in front of you and on the floor to the left will be some COG tags. Now just head back to where the barge dropped you to continue on with the on rails section.