The train crew must have already known that the girl would take a cup back to the stowaway child - the hot chocolate crew is so good and precise at their job that there is enough for every child - even when the girl hides a cup under her seat, she still gets a cup without the dancers losing their pace or wondering where the extra cup went.
The two hero children and the stowaway navigating through Santa's Workshop is accompanied by Christmas music from the '40s and '50s. What do the snippets that play from "Winter Wonderland", "Silver Bells", "It's Beginning to Look a Lot Like Christmas" and "Here Comes Santa Claus" all have in common? Listening to sleigh bells!
One may wonder how Billy has failed to believe in Santa Claus all his life, not once ever received a present from him. But a child growing up and living in an abusive home is unlikely to have faith in anything.
It's never stated that his parents abuse him. Still, an impoverished life might still leave one with little to have faith in.