The Silver Arrow by Lev Grossman narrated by Simon Vance
It's Kate's birthday and rich Uncle Herman, who wears a banana yellow suit and is a bit unorthodox, presents her with a most unusual gift: a train. Not a toy. This is a mammoth-sized steam-belching machine on a track in her backyard. When Kate and her younger brother Tom climb aboard, they begin an adventure that is beyond their wildest imaginings.
The Silver Arrow is a dream come true. Tom and a Kate create their ideal train with such delightful cars as a library car, a swimming pool car, and even a candy car. But the children soon learn that this will not be a pleasure excursion. They are tasked with transporting endangered species to less hostile environments. Kate is the conductor. She is charged with protecting and finding proper homes for animals such as a white bellied heron and a fishing cat. Yes, they really exist. I looked them up.
Good news: Uncle Herbert makes clear that more adventures await these siblings.