the cherry on the cake (or on top) A desirable feature perceived as the finishing touch to something that is already very good: the car is faster than a Ferrari, but the cherry on the cake is the price http://www.oxforddictionaries.com/definition/english/cherry

LIlya, you would be interested to know that the writer used "cherry" deliberately, so that an alliteration effect with "Choo" could be achieved. To understand what alliteration is, you can search the web.