Train Graveyard & Sector 7 - FF7 Walkthrough
When you land in the sewers, before you do anything else, equip Tifa with the gloves and armor you purchased for her in Wall Market and any Materia that you have for her too. I'd recommend at least two characters with Restore and at least one with Restore + All. When you're ready to continue speak with Tifa and Aeris to wake them up and fight the boss.
Aps is the boss you'll be facing off against in these sewers and the only attacks he has that are worth worrying about are his Tsunami ones (they hit your whole party with a tidal wave). He's also weak to Fire damage, so if you purchased a Fire Materia earlier you can make good use of it now.
Once the fight is over grab the nearby Potion then head to the northeastern section of this screen, grabbing the Steal Materia along the way. Up here you'll find a hole that you can jump down into to proceed to the Train Graveyard.
The Train Graveyard has a TON of treasures - a few are obscured and hard to see so you'll want to be thorough while exploring this place. Also you'll want to inspect the empty barrels that are found in this area too - almost all of them have some goodies inside.
When you enter this dungeon collect the Hi-Potion from the barrel north of the entrance, then the Hi-Potion from the top of the train. Go north along the top of the train until you reach the beam that you jump up onto. Go left and follow this to the ground. North of where you land you'll find another barrel with an Echo Screen this time.
Hi-Potion (Barrel near entrance)
Hi-Potion (Top of train near entrance)
Echo Screen (Barrel far north of entrance)
Potion (Inside Train car)
Potion (Very top of the first screen, before you switch to the next area)
Use my screenshot below and backtrack to where you see the Potion inside the train. You're going to want to collect the Potion then go south until you can go north again - then you'll come back up in the same area you see the Potion at the top of the screen. That completes the first part of Train Graveyard.
The second part of the Train Graveyard requires some moving trains around as well as more treasure hunting. Go left from where you enter the second screen to collect a Potion and an Ether. From there head northeast to the area where you see two trains that are a different color from the rest of them. What you're gonna do here is go inside the southern train first, it'll move on its own, then go inside the northern one.
After both trains are moved climb ontop of the roof of the stationary train that's in between them. Grab the Hi-Potion then climb onto the northern train's roof and head west. You'll find yourself back in Sector 7 with some serious shit going down.
Equip Cloud and Tifa with the Materia Aeris unequipped when she leaves your party and travel up to the top of the Sector 7 Pillar where Barret is. Speak with him to continue the story. When Barret joins your party again you'll have a moment to equip Materia on him and gear before the boss fight.
For this fight the only thing you have to know at all is when Reno uses Pyramid on one of your characters (it looks just like a hologram pyramid that floats overtop them) you'll need to break it with a melee strike. When a character is under the effects of Pyramid they can't do anything so removing it is very important.
After the fight catch a ride on Barret's back down to the Sector 6.