that also explains why EA could market “guaranteed seats” on the Air-Shuttle — they were able to roll out another airplane as a second section (even for one passenger!) because they only incurred the operating expenses (crew, fuel, landing fees, etc)
early on, when the Electras operated the hourly services, EA used Constellations for the extra sections; after DCA opened to jet operations and the Shuttle became a D9S operation, the Electras ran the backup flights