How to get Doomtrain in Final Fantasy 8 [Final Fantasy 8 Summon Walkthrough]
You need to assemble some items to get this. Get six Remedy+, six steel Pipes, six Malboro Tentacles and the Solomon's Ring and combine all of them. Doomtrain deal decent damage using Runaway train but the best part is it can inflict every status ailments to your target. It also has the Auto-Shell and Junk Shop ability.
If you want to know how to get all the GF view How to get all Guardian Force in Final Fantasy 8