To get Doomtrain you need to get the “Solomon Ring” item from Tear’s Point, it’s under the big statues. Then you will need to get six of the following items:

• Steel Pipes – Mug these from Wendigos around the Galbadia region or use card mod on Elastoid cards.
• Remedy+ – Use Alexander’s Med LVL Up ability, 10 remedy’s convert into 1 remedy+.
• Malboro Tentacles – Use card mod on Malboro cards (slow way) or mug/defeat Malboro’s in the Northern Esthar region, Island Closest to Hell or Island Closest to Heaven. The best way to defeat Malboro’s is to have Quistis in critical (yellow) health, give her interactive (Pandemona, Cactuar, Tonberry, Cerebus), and then use her limit break “degenerator” (use a black hole item, card mod a Gesper to get one) to instantly kill it as bad breath is friken horrible.

Once you have all of the above simply use the Solomon Ring to summon Doomtrain and it will join you party!