So...you are correct that Phantom Train and Doomtrain are not the same, but you are technically not correct that it is not already in the game. Doomtrain is the English name, but its original Japanese name is Glasya-Labolas, who is a boss in Syrcus Tower.

Not that they couldn't still add Doomtrain of course, we already have multiple Titans and other things from different games that were originally just different versions of the same character. Just thought I'd throw out that factoid.