https://na.finalfantasyxiv.com/dawntrail/patch_7_4/
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(Not needed to spoiler it because it's on the Lodestone and freely viewable)
Doomtrain was always hinted to be coming to the game since way back in 2015 when a 'wind up Doomtrain' was mentioned in a Rising event, and Yoshi later made joking comments that a then hypothetical journey into Garlemald would "be by Doomtrain" (paraphrased).
Besides, the 'FFVIII expansion' was technically ShB with Eden, GNB and all.
Can't wait to see what the trial is like though (I imagine Runaway Train is going to cause a complete wipe if mechanics aren't followed, in which case it would be fitting to afflict every debuff on the party, just like it does as a GF in FFVIII.) ;)
I’d like to see Monks get a special limit break on Doom Train that suplexes it.