I thought you summoned Diabolos through a ring. . .
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And you find the ring by defeating the train >.>
Orr, you find the ring at the train station when some shady character drops it
and then you get teleported to Diablos' world where doom train awaits you to
the added XIV tower where Diablos resides.
Don't test my imagination!
Oh wait, you don't get Diabolos out of a ring, you get him from a Lamp Cid gives you.
Watch
I want my own mountable Shiva, damn it. Why does Snow get twins when he's obviously the most undeserving FF character ever conceived?
In FFVIII, you gotta use a Magical Lamp Cid gives you to summon Diabolos. After you defeat him you get him.
In FFVI's GBA version, you get Diabolos after defeating Kaiser Dragon.
I think you meant the Solomon's Ring:
"To obtain Doomtrain, the player must have the Solomon Ring and at least 6 Steel Pipes, 6 Remedy+, and 6 Malboro Tentacles. Once all the items have been collected, Doomtrain will join automatically when Solomon's Ring is used in the inventory." -taken from http://finalfantasy.wikia.com/wiki/Doomtrain
Edit: Nevermind you found it ^^;