I think it was rather good of Bobbie to slip quietly away, saying over and over, "She doesn't want me to know she's unhappy, and I won't know; I won't know." But it made a sad end to the birthday.
* * * * * * The very next morning Bobbie began to watch her opportunity to get Peter's engine mended secretly.

And the opportunity came the very next afternoon.
Mother went by train to the nearest town to do shopping.

When she went there, she always went to the Post-office.

Perhaps to post her letters to Father, for she never gave them to the children or Mrs.Viney to post, and she never went to the village herself.

Peter and Phyllis went with her.