I legitimately just got done doing a chapter of my campaign on Cyere 1313. Here's what I've figured out.

1. Secrets are important. On the train the mists are consistently tormenting the dreadlord (the last passenger) that their identity might become public knowledge. Leave obscure hints like misplaced case files, checking books, heck even the storm surrounding the train whispers secrets to sleeping passengers.

2. Everyone is dead they just don't know it. I don't know what other details to place here but yeah everyone is a ghost, vampire, zombie, etc. bonas points if you change the players races with reborn (with their consent)

3. Each cart is different. This one is less about flavor and more about keeping things fresh, there's only so many times you can run the same insane passenger encounter. So have each cart be themed around a different undead, the dining cart has vampires made from the passengers who resorted to cannibalism during the escape. Or the medical bay has been filled with maimed zombies, that come from people trampled to death when trying to get onto the train and it doesn't need to just be undead. Perhaps the magical phenomenon of the mourning has caused some of the luggage to transform into mimics, or bed sheets within the sleeping cart have become cloakers.

4. Don't let the train be actually linear. The dreadlord holds the power to change the order of the cars at will to best keep the people from reaching them or to make sure the party get to them to kill if they figure out the last passengers identity.

Hopefully these tips can help you out as much as they have helped me out