You're going to start the next part of Killzone 2 with a pistol, so if I was you, I'd start looking for something with a little more bang for your buck. You're in luck at this point because the developers have had pity on you and have put a Boltgun in a rack to the left of the resting tank. Once back on the outside area of the train, mount the gun turret of the stationary tank and send the incoming enemy to their makers. Always make sure to use the easily recognizable and handy explosive barrels and Petrusite containers to best effect.
You'll be up against a Helghan tank in the next area of Killzone 2, so put your grenades away. The wind in this part can actually blow them back at you. Instead, grab the Missile Launcher in the rack and scrap the armored caddy. Once you and Rico reach the fence, you have to give Rico a hand over and then you'll be by yourself. Grab the Helghan symbol stuck to the side of the train as you're exiting the platform area. After reaching the Lower Refinery Complex, use the waypoint marker to find the fuse box and then blast it to disable the electric currents. Now, clear the area of the enemy. Use the lift to descend.
ArcTrooper Boss will challenge you by himself, so you'll need to use the elements in the environment to help take him out. Notice the handy moving transport containers moving across the ceiling. Lure ArcTrooper Boss close to one of them and then shoot it down. ArcTrooper Boss will be stunned for a few moments by the Petrusite discharge, revealing the three red tanks on his back. Target the tanks before he can get himself back together. Defeating ArcTrooper Boss will mean you can pick up the VCS Electricity Gun. This weapon reloads by harnessing the natural electricity in the environment, so it's a nice weapon to try out for awhile.
Expect to be the target for a few Helghan as you ride the bucket on the conveyor belt. Once again, always make use of the explosive containers to eliminate targets (when available). Rico will notice the electricity field ahead. You have to use the waymarker to quickly find the fuse box and blast it before you become fried bacon.
You'll find Sentry Bots will be deployed when you reach the bridge. While challenging, they are not as deadly as the ATAC. They do tend to attack in greater numbers, which is a pleasure and a pain. Take your time moving through this area at a steady but not hurried pace and they should attack in numbers of two and thus be easier to eliminate. Note: If you don't believe me, try running through as fast as you can, but be prepared to be overwhelmed. Once you make it to the other side, ride the lift.
Another set of Sentry Bots will show up once you reunite with your squad. Move to the higher areas so you can eliminate the enemies above while mowing down the bots. Once you reach the top, press the button in the control booth to send the lift down for Evelyn and Garza. Suppress the enemy at the Landing Zone until Evelyn and Garza show up. Enter the Comms tower with Evelyn and make sure to pick up the sniper rifle at the top. Protect yourself and Evelyn from the enemy below as Evelyn uses the radio. After the LZ is cleared, just meet up with your squad. At this point, you have completed the Tharsis Refinery section. Continue with part 5 of this for the final sections of the game.