Boston, New York, Philadelphia, ____ and Washington DC are the main cities in the first group called a megalopolis (9)

I believe the answer is:

baltimore

(Other definitions for baltimore that I've seen before include "Largest city of Maryland, USA" , "Port on Chesapeake Bay" , "Large city in Maryland" , "City of north Maryland" , "City north-east of Washington DC" .)

I've seen this clue in The Times and The Sunday Times.

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