This is an interesting assignment, but I'm not sure what your question is. As a senior in high school, I'm sure you've done some work with both imagery and narration (telling a story), so you should have no problem writing this descriptive essay. Your teacher asks you to pay special attention to the sounds, but effective descriptive writing will include all kinds of imagery.
No one here will write this for you, but I can help get you started. Begin by envisioning what a scary train ride would be like to you. What would you see, hear, feel and even smell on this ride? Once you determine those scary (and perhaps not so scary) elements, what would the basic train ride sound and feel and look like? Make a list so you don[t forget to include them when you write.
On another list, imagine all the sights and sounds which might occur upon discovering the train you're riding on is somehow haunted. Would you scream, suck in your breath as if punched in the gut, stay silent in hopes of making sense of the noise? Whatever best suits you and your approach to this scary ride is what you write down.
From there, it's a simple matter of telling the story you just envisioned. There are few real guidelines here, just tell your story using effective description--especially those sound images. Try to avoid images that are too cliche (old and overused, like a ghost which looks like someone in a sheet, or the words "scared to death" or "white as a ghost") so you'll sound fresh and interesting. Happy writing!
Is this a prompt given to your class by your teacher? I really think this question will best be answered by YOU - as only you know how you react to fear. Perhaps - for you - there is no such thing as scary enough, in which case, your answer will not be typical. Maybe for you - riding a really scary ghost train is a laughable experience because you do not buy into the idea that ghosts are real.
That would be my immediate answer - anyway, if I was given the same prompt. However, if I knew the bigger picture in the assignment was to explore imagery (sound) and emotions associated with fear (which is likely close to the point of this assignment) - I'd conjure up a similar experience in my head - but try to imagine real reactions to genuine fear. I know when I wake up in the middle of the night - in the stupor of sleep - and think I've heard an unusual noise in my house - it seems like everything is twice as loud in my head than normal. Since we're only paying attention to sounds here, I always notice my ears ringing when I'm actually afraid - and things sound cloudy - like they are happening in a tunnel. Also, although I probably cannot hear my heart, I can always feel it in my temples and lips - which almost resonates like sound does. I'm not usually aware of the sound of my breath, but I'm sure in times of actual fear - I don't actually scream out - instead I just get really quiet and my breathing gets quicker.