Update! Graffiti Games posted the following on July 15, 2022:Good news everyone! The achievements issue on the Xbox version of TURNIP BOY has finally been fixed! 🎉
And as always, let us know if you're still experiencing any other issues!
Thank you everyone for your patience throughout this process as we tackled the fix.
Huzzah!
Note that several people have reported having to uninstall/reinstall the game or hard reboot in order for the achievements to unlock. Personally I was able to clear this one no problem but until I did a hard reset, I couldn't walk back to the greenhouse without the game crashing for the Nuke achievement.
Once you have beaten the game, the Limitless Line (endless train) opens up across from the sub shop in Layer Lane. This is an endless roguelike mode moving from train car to car fighting enemies and bosses. About 8 rooms in is a boss fight with Conductor Onion, which you have to beat once for another achievement.
Once you've defeated Conductor Onion once, and revisit the train, there's a chance for ??? to spawn in a room on the train. It's a relatively small chance, so you'll probably have to visit the train several times to get it to appear. Once you have defeated ??? the achievement will unlock.
Some have reported meeting ??? before Conductor Onion, so it appears that is possible as well.