This part can be very confusing if you don't know what to do. If you die here you will have to go all the way back to the beginning of the train. The blue health bar is the coolant system.
The coolant system is controlled by two switches in the lower inside part of the train. You will want to keep this from going low, as the train will start to catch on fire. There is also a T-ENG replenishing machine in this room and a turret which drains your T-ENG.
By now you should have noticed huge bullet casings everywhere. Go up on the outside of the train, towards the huge cannon. Behind the cannon you will see a bullet delivery system. Basically, what you need to do is grab a shell, go to the side, and load the shell. Then you need to energize the shell by hitting another switch on either side. You can have three shells loaded at one time. Once you energized them you have a limited amount of time to blast it at the Akrid. If you're running low on shells, you can use the crane near the back to load shells.
The goal here is to blow up all the round orange spots on the side of the boss. Once these are destroyed you will see a message that says "final shot". Just blast him in the face and call it a day!
With all that said, I recommend ignoring the turret all together. It's strong, but just way to slow. There are four turrets on both sides of the coolant room that can be used to do decent damage. Have one or two guys down there running the coolant and shooting the turrets. There is also a helicopter near the back, so have one guy in that for sure. Make sure the guy in the helicopter calls out which side Red Eye is coming up on as the guys in the coolant room can't really see.
The video below was done on hard but the same strategy applies to Extreme.
You may want to see Flaffy Quak on youtube he is very good at finding the ammo fast, but to make the railway gun stronger and 10x faster play by yourself 4 some odd reason in solo the railway gun turns mad fast§ (my friend tried 2 help me and we lost 3 times when it was just me on hard alone I beat him 1st try & didn't even have 2 repair the train) hope you beat him!
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