Instructions
Play through the game's main storyline until you reach the boss battle against Edea at the end of Disc 2.
Select the "Draw" ability at the beginning of the fight and select "Alexander" from the draw menu. Defeat Edea using your best GFs, attacks and magic abilities.
Travel to Tear's Point in the Esthar region via airship or Chocobo, or on foot.
Open the treasure box near the Draw Point in Tear's Point to receive the Solomon Ring. This is a necessary item for unlocking Doomtrain.
Travel to the wasteland areas that surround Galbadia Garden to encounter Wendigos. Use the Steal or Mug ability on a Wendigo to receive a Steel Pipe. Continue this process until you have collected six Steel Pipes.
Travel to Grandidi Forest in the Esthar region to encounter Malboros. Use the Steal or Mug ability to steal Malboro Tentacles from a Malboro. Continue this process until you receive six Malboro Tentacles.
Travel to any item shop and purchase 60 Remedy. Use Alexander's Med LV Up ability from the GF menu to turn the 60 Remedy into six Remedy+.
Open your item menu and select the Solomon Ring after collecting six Remedy+, six Malboro Tentacles and six Steel Pipes. This will give you the option to summon the Doomtrain GF.