Rachel dies during transport on the train because she could not breathe due to asthma.
When Rachel first tells Hannah that she is going to be her best friend, Hannah replies that she already has a best friend at home.
It is not until they arrive at the concentration camp and Hannah is in the showers that she realizes that Rachel is dead. She asks Esther where she is, and Esther just keeps singing.
Rachel always had trouble breathing in the spring. (ch 11, p. 92)
Hannah remembers the "peculiar" way Rachel used to speak.
While traveling in the train was difficult for most of them, Rachel's death demonstrates how quickly anyone with health limitations could die. Poor Rachel never had a chance.