Novel by dduane; actually, second in a series. (The first is "Book of Night with Moon"; the third is "The Big Meow", which she is writing on and off as a fan-funded publication as she can't interest her publisher in it.)

It's about wizardly cats who are in charge of the inter-planar gates that exist under major subway stations. You can see how this is relevant. ;) Same universe as the Young Wizard books.

I think galaneia has a copy of each if you want to borrow them.