Character mistake: The narrator says that Thomas had never delivered the mail before, but he did in Season 4.
Other mistake: Mavis and Cranky should've realised that Thomas woke them up the first time he blew his whistle.
Continuity mistake: Thomas says that Mavis and Cranky never make mistakes, but they have in "Mavis" and "No Sleep for Cranky."
Answer: Breakdown trains help the engines back on track.