Mount Corel Huge Materia - Disc 2
*Important* For this part of the game you have to collect a Huge Materia by stopping a runaway train before it hits the town of North Corel. It's possible for you to fail this and miss the Huge Materia - if you do then you will not be able to get the Bahamut ZERO Materia later on in the game and a decent chunk of the story will be missing too.
For this reason I recommend you save outside of North Corel before even attempting this - if you fail then repeat it until you're successful. Also take the time to equip yourself appropriately - you're going to want characters capable of healing the group and doing burst damage, preferably without using Summons since we're going to be timed.
If you've forgotten where North Corel is - just fly around and look for the Gold Saucer, it's within a stone's throw distance. Skip right on through North Corel when you arrive and out the northern end of the town - to the area where the long wooden bridge is (pictured above).
Run all the way back to the Mako Reactor where two Shinra Guards will be waiting for you. After defeating them in battle you will be thrown into the timed event so make sure you're ready. It begins with you having to alternate two levers in a train to catch up to the one that has the Huge Materia. When you jump onto that train you'll be forced into 5 different battles, the timer will continue during all of them.
If you have Enemy Skill Aqualung I would recommend just spamming that on two or three characters for the duration of this quest. It's a very fast casting attack that deals 1500+ damage each time. It'll make quick work of basically every enemy.
The fourth battle you get into against the Eagle Gun enemy (pictured below) you'll want to try and steal the Warrior Bangle accessory it has. This is an item which can only be obtained from this enemy and offers you a good amount of protection. This item isn't so good though that you shouldn't pass it up - if you keep trying to Steal it and aren't having any luck don't feel bad about giving up.
After the Eagle Gun the final 'boss' is a single Shinra guard. Once he's out of the way you'll now have to bring the train we're on to a halt.
How to stop the train:
Up + Triangle
Down + X
You have to do Up + Triangle then Down + X then Up + Triangle and keep repeating this until the train stops. Your companions will yell at you that you're messing up and accelerating the train, which is what is supposed to happen. If you do this correctly before the time runs out then you'll successfully stop the train and complete the mission.
Back in North Corel you'll receive the Key Item Huge Materia, speak to the townsfolk to also receive the Ultima Materia. You'll get to spend the night at the Inn for free then you should swing by the building just south of the Inn to receive Barret's Level 4 Limit Break, Catastrophe.
Our next destination is Fort Condor, a tower near Junon that's part of one of Final Fantasy 7's main Side Quests. If you've never been here before I recommend reading more about the Fort Condor Side Quest before continuing with the guide.