You need 666 items,

6 steel pipes
6 marlboro tentacles
6 remedy +

To obtain doomtrain, you must need these items including the solomon ring. The solomon ring can be obtain from the lunatic pandora tear's point.

The steel pipes:
Mug a wendigo in one of the platteu in galbadia area

Marlboro tentacles:
Go to island closest to hell and beat a marlboro.

Remedy + :
You can only have a remedy + if you have alexander and your gf has learned lev med.