Both are correct, but using "the Routemaster" specifies a particular type of bus, while using "a Routemaster" indicates any Routemaster bus. So, it depends on the context in which you are using the sentence.
Both are correct, but using "the Routemaster" specifies a particular type of bus, while using "a Routemaster" indicates any Routemaster bus. So, it depends on the context in which you are using the sentence.