Years ago, Eastern Airlines operated a "New York - Boston" shuttle service each hour from about 6 am to 9 pm (roughly). I involved aircraft going both ways at the same time. Each flight lasted for about 90 minutes, so there were multiple aircraft in the air at the same time, some going one way, the others the opposite way.

Anyway, I was in New York and headed for Boston. There were no reservations, only a first come, first served queue. When I got to the ticket counter, I was told that the last aircraft was full, so I requested a seat on the next plane. [quotes not exact] "That won't be necessary," said the clerk. "We can move out another aircraft for you." I was stunned. "How can Eastern afford it?" I asked. "We can simply fill up the aircraft with cargo to its weight limit, and we can carry more cargo with fewer passengers, so it all works out."

So, I was led aboard an aircraft along with two pilots and two stewardesses. I had positively the best passenger service EVER.