This was a few years after the Great Depression, and the railroads were then ready to invest in new streamlined trains of lightweight construction. Most railroads opted for the new diesel technology, for locomotives powering the streamlined trains. Although some railroads continued with steam, with some steam locomotives even receiving streamlined shrouding to match the new cars.
These passenger train modernization programs continued into the early 1940's, with most railroads introducing streamlined equipment on their most important trains. But the United States entered World War II in 1941, and any manufacturing capabilities were then directed primarily towards the war effort. And the modernization programs would resume after the end of the war, in 1945.
In 1945, another new innovation came from the Burlington Route, the dome car. And dome cars became popular on many trains serving the most scenic parts of the country. Except for many trains serving the northeast part of the country, where various tunnels had inadequate clearances.
The Burlington Route was also the last railroad to completely reequip a major passenger train, when new passenger cars were acquired for the Denver Zephyr in 1956. But Interstate highways and air travel had already started cutting into passenger train ridership. And passenger trains were discontinued, consolidated, and shortened. And surplus streamlined cars became available to replace most of the remaining older cars.
In 1971, Amtrak was formed, and initially acquired from the railroads approximately one third of the intercity passenger cars previously in service, all streamlined cars. These were considered the "best" third of the passenger cars available. But most of them were still twenty or more years old, and overdue for replacement. And despite funding uncertainties throughout Amtrak's history, Amtrak did manage to do what the money losing private railroads were not in a position to do, acquire new equipment.