AZL 73550-8 Amtrak Diner Lightweight Passenger Car | Phase IVb #8507

Lightweight passenger cars required developments in steel processing that were not available until the 1920s and 1930s. Lightweight passenger cars enabled the railroads to operate longer passenger trains; the reduced car weight meant that more passengers could be carried in a greater number of cars with the same locomotives.

The cost savings in hauling capacity coupled with the increased car type options led to the quick replacement of heavyweight cars with lightweight cars.

Wikipedia: Lightweight vs Heavyweight Cars