Fixed-rate fares and unreserved coach seats are why railways and intercity bus lines kept older equipment around to handle surges in ridership.
There were promotional fares that tried to achieve what yield management does but being fixed in themselves they were risky. My brother rode the Mainstreeter on the half-price Youth Fare in January 1967 and across Montana college kids were sitting on their suitcases. There was no immediate way of halting sales.
I agree, but one thing did sort of ruin that idea of having extra old running stock. My grandparents wanted to go to Colorado Springs for Christmas to see one of their children's family. I planned to ride as far as KC from St. Louis on the Missouri Pacific.. I made the reservations plenty far out to get hopefully a nice sleeper for them since it was one of their first pullman trips. When we boarded the train however they did have wonderful new well maintained pullmans in the consist, but the car we got was a very old version with wall lamps that hung out from the walls and other things you could tell meant it was probably from the 30s or maybe 40s. Our trip was in the early 60s. The car rode rough like it had a flat wheel and was noisy at that. When I got off it was late evening an later they told me it was impossible to sleep in the car it bounced and made so much noise. So extra cars are nice but it was too bad that early booked customers got stuck with the old equipment.