Why did you make a level design before you knew what you wanted it for?
Anyway, if you want to not have to animate any of the NPC's, how about this: early on, have the train running, with most people asleep (do small animations like breathing, tapping toes, looking on phone, etc. so it doesn't look like a traincar full of mannequins). Then, you notice that you start going through a tunnel that has odd stone and strange symbols etched into the walls. Suddenly, the train moves into a tunnel of some strange-colored light, which quickly becomes blinding. As soon as the light fades, you find yourself travelling through some strange cavern instead of a subway, and everyone on the car has vanished. You then proceed through the cars, solving strange cryptic and supernatural puzzles to allow you to move forward and find the secret of what's happening... HINT: It was a derailment... you died and are in hell.
I actually figured I'd go one better and I asked ChatGPT for you:
"I have a 3d asset of a subway car. I'd like to use it in a horror game, but I don't want to have to make many character assets or animations, so I'd like an idea for a horror game that takes place on a subway car with no other people on the train."
"Here's an idea for a horror game that takes place on a subway car:
Title: "The Last Train"
Plot: The player takes on the role of a subway passenger who is the only person left on a seemingly endless subway journey. The train keeps passing by ghostly and abandoned stations, and strange things start to happen. Doors randomly open and close, the lights flicker, and the player hears ghostly whispers and footsteps.
Gameplay: The player must navigate through the subway car, solving puzzles and finding clues to uncover the mystery of what happened to the other passengers and why the train is stuck in this never-ending loop. The player must also avoid supernatural entities that haunt the train and try to stop the player from uncovering the truth.
As the player progresses, they'll discover more about the history of the subway line and the events that led to the train being stuck in its current state. The player will have to use their wits and quick reflexes to survive and finally reach their destination.
The subway car asset you have would be the perfect setting for this kind of game, as the tight and confined spaces of a subway car can add to the sense of unease and horror."