The Circus Train by Amita Parikh

Lena is a lonely young girl who lives and travels the circus circuit with her father. Her father is one of many eccentric performers. As a young child she got polio and as a result of it is confined to a wheelchair resulting in her father being over protective yet loving. 

This takes place in Europe before and during the start of WWII.  A young orphaned boy with a dark past and a secret that could result in his death because a part of the circus act. He enters Lena's life changing how she views herself,  giving her more confidence and developing her arc.  The character build is slow and so is the storyline. It doesn't really develop and get interesting until half way through the book. The best part is the last quarter of the book. 

Expect betrayals and secret reveals.  My largest complaint is that no matter how much I tried, I could not figure out the layout and logistics of the Circus, which lead to me wasting a lot of unnecessary time trying to figure it out.