Just thinking about this.
When I was a kid, in the 1950's, watching trains at the Fullerton station, there were basically three passenger trains: The Grand Canyon; No.7, the Fast Mail & Express; and the multiple daily San Diegans. The San Diegans were all stainless steel cars, mostly Budd. The Grand Canyon had a mixture of heavyweight and lightweight cars. The mail train was almost all heavyweight, with some boxcars and a small number of lightweights.
The heavyweight cars on those trains were built in the 1915-1935 era, making them 20 to 40 years old. The San Diegan cars were older than I imagined, all being built between 1937 and 1941, already 15-20 years old, although I innocently thought they dated from around 1950. That made them only 5 years newer than the late heavyweights, and 25 years newer than the older heavyweights. Lightweight cars on the Grand Canyon dated from 1938-1948, making them 7 to 17 years old.
There were still a few automobiles on the road from the mid-1930's to then modern times, but most from before 1935 were too obsolete to be used any longer. So, 20 years was the age of the oldest autos, with a few exceptions.
The passenger car shops had maintained all of those cars, repairing cracked frames, broken trucks, steel side panel replacement, and wreck damage. They air conditioned the cars built before that feature became standard. And the cars all rode well, looked fine, were comfortable, and had reliable heating and cooling.
Today, all Amtrak cars are streamlined, but they are longer in the tooth than most of the equipment that I watched in the mid-1950's. In 1976, I was down at the yard office, checking my position on the extra board, when 4 F40PH's and about 25 Amfleet cars pulled up and changed crews for Los Angeles. They were all brand new, heading west to replace the weird collection of cars on Amtrak San Diegans, which included former Burlington Budd combines and ex-SP Daylight articulated chair cars. That was 49 years ago.
So, that puts the oldest Amfleet cars at about 50 years of age now, and the Horizon and Superliner cars are, what, between 20 and 40? Now, they have reached the age of the heavyweight cars of my youth, but there is a big difference in the amount and quality of the maintenance they have received. Some are wearing out, others have deferred maintenance and sit in the weeds, awaiting a day when they might be repaired, and others soldier on, with varying degrees of good repair. Of course, there are some newer sleepers and diners, and some medium distance coaches on the state-supported trains, but the passenger car fleet overall is not very new. What now?
It appears that the Amfleet cars were one of Amtrak's best investments in equipment. They just keep going, and going.
