The first of the Huge Materia you need to find lies in the abandoned reactor by North Corel. Go to Corel, and head back into the mountains. (Just go back the way you got there in the first place). Just as you arrive at the reactor, you'll see some Shinra employees take the materia out on a train. Cid will run in and find a saller train. Now, you have to make up for lost time by pushing the buttons as fast as you can. Try putting the controller down and use both hands--it helps. It should not take you any longer than a minute to catch up to the other train and hop across. Remember, you're battling the clock here, and some of the battles ahead can turn into long ones if you aren't careful. Lightning magic works the best against these mechanical enemies. You can (if you want to) steal the Warrior Bangle from Eagle Gun, the last enemy you'll face on the train. It's great armor, but it doesn't grow your materia. Anyway, when you get to the front of the train, kill the engineer, then mess with the buttons all you want. It doesn't matter if you play with them or not, because if you kept within the time limit, the train will stop of its own accord. If you didn't get to the controls in time, though, the train will crash into North Corel, you will lose the Huge Materia, plus all the stuff the townspeople give you later. Speaking of the townspeople, if you're successful, that kid will give you his glowing stone, which turns out to be the Ultima materia. Also, the person in the tent on the west plateau will give Barret his level 4 Limit Break, Catastrophe. Btw, if you do lose the event, you can still get Ultima--for 50,000 gil.