Found: YA Book about a secret world that can be found by going down a hidden set of stairs in a city

I hope someone can help! I read this book around 2000-2003. It seemed brand new at the time. I LOVED it! From what I recall, there was a hidden fantasy world down a hidden staircase, and the characters who lived down there were trying to keep the world a secret but someone in our world was trying to uncover it. I believe the secret set of stairs was in a train station but I can't seem to recall this particular detail. It's not the Secrets of Droon!

The Secret World of Og?

Reminds me of The Secret of Platform 13, though I don't know that there's actually a staircase in the train station, but definitely an opening to another world!

OMG! That's it!! I can't believe you solved it! Thank you so much! I honestly couldn't remember very much at all about the book (which is odd because I was obsessed with it as a child).

>4 ChelsieH2019: Chelsie, you've got two suggestions. Which one was it?

>5 Bargle5: The Secret of Platform 13!

>4 ChelsieH2019: >6 ChelsieH2019: Glad I could help! It might be time for me to do a reread -- it was one of my favorites in the early 2000s, too.

>6 ChelsieH2019: Thanks.