Concept: The train acts as a vessel to carry departed souls to the afterlife. The great number of lives lost at Cartineau have coalesced into this bizarre phenom, causing disturbances in the rail lines used by Amajina & Sons to transport minerals to Thanalan for processing and resale.
Dungeon Layout
Boarding Platform (leads to second class car)
[Engine]-[Baggage Car]-[Sleeper Car]-[Diner]-[Lounge]-[First Class]-[Second Class]-[Coach Car]-[Caboose]
The party would spawn on the boarding platform, where they fight the first pack of mobs. Upon defeating them the train doors open, allowing the party to enter. Once the full party has zoned in, the zone line closes for the rest of the instance.
The party enters into the second class car. The door leading to the first class car is locked and requires the party finding the switches to unlock the door. Thus the part would have to work their way through the second class and coach cars on to the caboose, where the first boss will be waiting.
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Boss: Brakeman
Abilities
Spectral Embers - Summons a fire sprite that fixates on one player. Explodes upon reaching their target, damaging the whole party. Become Tiny Embers when doused with water.
Charge - Runs to target, dealing damage and stunning them for 3 seconds.
Restrain - Hurls a chain at one party member, immobilizing them. Breaks if enough damage is dealt to the chain's Weakest Link.
Player Abilities
Douse - Ground-targetted AoE. Can only be used by interacting with the water barrel.
Mechanics
Mitigating the explosion from Spectral Embers is key to surviving this fight. Once the boss is pulled a water barrel will become targetable. If a player clicks on the barrel, they switch into a vehicle mode with the only action on the pet bar being Douse. Using Douse on the Spectral Ember will turn it into a Tiny Ember, which can then be killed by DPS before resuming on the boss.
Brakeman will use Restrain on random party members. It'll usually be whoever is the target of the currently-summoned Spectral Ember. The chain between Brakeman and the restrained player has a targetable part called Weakest Link, and once the Weakest Link is "killed", the effect will end.
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After Brakeman dies, the party finds the First Class Key, which can be used to unlock the First Class car. Defeating all the mobs in the First Class car opens up the door to the lounge car. The lounge car is gated by items that need to be placed in the proper spots for the path to open up. The items are portraits of the four characters that were in the original train in FFVI (Cyan, Sabin/Mash, Shadow and The Ghost). Each portrait would have a name, like "The Dark Mercenary" for Shadow, "A Spirited Monk" for Sabin/Mash, "A Morose Samurai" for Cyan and "Well-traveled Soul" for the ghost.
Moving on to the diner car, the second boss fight is found there.
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Boss: Steward & Stewardess
Steward Abilities
Skull Crack - Stuns target for 4 seconds.
Hard Swing - Knocks target back 5 yalms. Will only use on targets near or attacking Stewardess.
Hexa Strike - High damage attack. Places a Vulnerability debuff on target. 3 second cast time.
Cover - Will absorb physical damage aimed at Stewardess. 8 second duration.
Stewardess Abilities
Leg Shot - Decreases target's movement speed by 30%. 10 second duration.
Toxic Sludge Bomb - Places a patch on the ground that deals damage over time to players standing in it.
Leeching Ammo - Lasts 6 seconds. Causes any attacks to drain HP.
Recovery - Recovers HP. Will only be used if the difference between Steward and Stewardess' HP is more than 10%.
Mechanics
Steward and Stewardess will attack individually. While Steward has an aggro table, Stewardess does not. The main gimmick of the fight is that both need to be brought down at around the same time, as a difference in HP between them greater than 10% will cause Stewardess to effectively bring both of them back up to full or near full HP by spamming Recover.
Cover and Leeching Ammo add difficulty to the fight, as the amount of damage taken from Cover and HP recovered via Leeching Ammo may bring either or above that 10% HP difference.
Steward is there to inconvenience the party via stuns while also protecting Stewardess. There is some synergy between their skills, as Hard Swing may be set to be used on the tank just as Stewardess uses Leeching Ammo. Another example would be using Skull Crack just as Stewardess creates a patch of Toxic Sludge under the stunned player. Yet another example would be Hexa Strike being used on a target affected with Leg Shot.
As a visual cue to show the difference in HP between the bosses, once the bosses' HP difference is more than 5%, a green tether will form between them. Once it hits 9% the tether turns yellow, and at 10% or more the tether turns red.
Intended Strategy
Tank would have to pull Steward close to Stewardess, with Stewardess focus targetted. This is so that the tank can stun Stewardess when she uses Leeching Ammo. In the event Steward pops Cover, the tank needs to pull Steward away and keep him more than 5 yalms away form Stewardess or order all DPS to stop on the bosses until Cover expires.
Healers would have to make sure to Esuna/Leeches the effects of Leg Shot while healing damage from Stewardess. In the event a player is stunned and has a sludge patch under them, the healer needs to make sure to heal them while also keeping an eye on the tank's HP.
DPS need to know when to back off in order to not cause the bosses to heal themselves by paying attention to the tether color between the bosses. I do realize that you may get DPS trolling groups by purposely making the bosses heal themselves, though...
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After clearing the Diner, the party can move on to the Sleeper Car. A large puzzle composed of three pieces of a portrait are laying about. Whenever each piece is picked up a pack of strash mobs will spawn. The three pieces when placed inside the frame by the door will create the portrait "The Greatest Swordsman" (yes, this is a reference fo Seigfried, who is encountered by the party in the original Phantom Train), unlocking the door to the Baggage Car.
Baggage car has a pack of trash and the remains of a tent that players cannot interact with.
Crossing into the engine, the boss fight will start.
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Boss: Phantom Train Engine
Abilities
Steam Burst - Damage to one target.
Scalding Steam - PBAoE that damages player and anyone within 3 yalms of the target. Removes Unliving Fire. 3 second cast time.
Unliving Fire - Marks player with Unliving Fire, causing them to take damage over time indefinitely. Can only be removed by Scalding Steam.
Locomotive Chase - Ejects one player from the top of the train on to the rails in front of the train. Increases running speed by 300%. Lasts 10 seconds.
Mechanics
The phantom train engine can be compared to Demon Wall from Amdapor Keep in that the fight is on a sort of time limit and it will ignore aggro tables for the most part. Only the train's smokebox (the chimeney thing in the front of the train) will take actual damage, and the encounter will take place on top of the train itself.
The engine will use Steam Burst on random players. The main mechanics involve dealing with Unliving Fire and being ready for Locomotive Chase. Unliving Fire will deal considerable damage (it's stronger than any normal DoT effect). To remove Unliving Fire, affected players need to be hit with Scalding Steam. The person targetted by Scalding Steam will have a small cloud of steam at their feet for 3 seconds before the ability goes off and deals damage.
The train will randomly select a player and eject them on to the tracks in front of the engine. From the moment the player hits the ground, their running speed is increased by 300% (thus giving them the chance to run away from the train as it tries to run them down). While the train tries to catch up to them, it will have 5 stacks of Locomotion, and will lose one stack every two seconds. Each stack of Locomotion increases damage dealt by the train's inpact to that player by 40%. Once the train comes in contact with the player, the player will take damage, be swept up by the wind currents around the train back to the top to rejoin the party.
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Instance ends when the train is defeated.