I'll just add a few comments here to others. Eastern pioneered the Shuttle in 1961, on a route which up to then had been dominated by American and Northeast. Various carriers had the licence for the route, generally run as an add-on to longer distance trips, and it's indeed surprising how it has gone from carrier to carrier over time.

Reading about Eddie Rickenbacker, Eastern's longserving chief, you can sort of see how a serial aviation type like him would be attracted to the idea. Main driver was the plummet of their Lockheed Constellation values as Eastern brought in the DC8 and the Lockheed Electra at that time - there were spare aircraft available. It certainly made them dominate the market for the next 25-30 years.

British Airways brought in a comparable Shuttle operation on the main UK domestic trunk routes from the mid-1970s, based very much on the Eastern ideas, and I've spoken to one of their onetime ops personnel about it. They too had an abundance of aircraft at the time as a main driver, with the older Tridents, which formed a separate Shuttle fleet. There were a lot of similar myths, and fascination by the media. Firstly there was never a flight with one passenger, that was a tremendous marketing story, but the minimum load they ever took was seven. The backup ran 10 minutes behind the main section, and by the time it was ready to go passengers had started to turn up for the next hourly departure anyway. With an hourly operation passengers, certainly later in the day, tended to be just "turn up and go", rather than heading for a specific flight. In addition, once there was a planeload on the first section, it would be sent off, instead of waiting for departure time. BA followed the Eastern style of pay-on-board. I never actually did this, nor seemingly did most others, as I was always provided with a ticket by our travel organiser, but in days before universal credit cards they certainly took cash on board.

Key was a considerable prediction routine for loads on each flight, based on history, major events, and such like. They estimated each flight in advance, and were pleased how close they got, and then did an equivalent post-review afterwards. They were pleased with how little flight waste there was. This improved further as additional routes were added, just like the Eastern operation which did both La Guardia and Newark to both Boston and Washington.

The thing that really killed off both operations was a fundamental change in aviation, firstly the widespread adoption of yield management, different fares for different flights booked at different times, which the Shuttle could not do. Further, as the airports became full, and slot restricted, having slots available for a Shuttle backup became wasteful, they were better used for regular flights to other destinations.