Best Engine Ever

S19, E16: When Emily breaks down and disrupts a race between Connor and Caitlin, Caitlin shunts Emily to the castle causing Emily to travel faster than she ever has been before. Emily is in awe of Caitlin, who she believes is the best engine ever and becomes disheartened at her own ability. But when Caitlin's brakes fail, Emily races to save her from an accident proving that she too can be a Really Useful Engine.