YA Sci-Fi from ~20 years ago

I read this book in the early 90s when I was about 11 years old. My only recollections about the story are these:

A girl protagonist
Lives in a mysterious city with doors connecting to other time periods
A boy in her class gets stuck in the past and grows up to be Leonardo da Vinci
There is a sleeping giant under the city who must not be awakened

I believe the book begins in a train station with the girl arriving in the city.

Thanks!

This could be A Tale of Time City by Diana Wynne Jones. The protagonist is being evacuated by train during the Second World War when she's transported through time.

That's the one! Thank you, konallis.