Pullman began building 80’ wood “Palace Cars” in the early 1890s; these included sleepers, dining cars, parlor cars and observation cars. Similar to the old HO “Pullman Palace Cars” kits once offered by MDC/Roundhouse.
Generally speaking other cars (coaches, baggage, RPOs, combines) would be shorter, like 70’ coaches and 60-67’ for baggage or RPOs. This continued to be true for heavyweight cars; lightweight streamliner cars introduced in the mid-late 1930s were often all 80’ long.
Not sure about the exact date, but 1910 is about the time the US Postal Service required that all Railway Post Office cars be all steel. Before that for a time they required that wood cars have a steel underframe. This meant that a train could often have a steel RPO with all other cars being wood.
Since the RPOs had to be steel because steel was safer in a wreck, passengers started to complain that they wanted the added safety afforded to RPO clerks, so railroads began buying heavyweight all-steel passenger cars (and rebuilding some woodsided cars that had steel underframes to being all-steel).
As Dave points out, some wood cars continued for some time after 1930 on commuter trains, as emergency fill-ins, and as “Jim Crow” cars in the South. But when the Depression hit, and rail traffic declined, railroads primarily used their most recently purchased equipment, so you’d usually see only heavyweight cars on a train during the 1930s.
Heavier all steel cars required heavier engines. In 1910 a 4-4-2 or 4-6-0 could generally pull a train of wood cars, but a train of heavyweight steel cars the same length would be too heavy. Railroads shifted mainline passenger trains to using 4-6-2s, 4-8-2s, and then 4-6-4s and 4-8-4s.
For example, Great Northern’s wooden-car Oriental Limited used 4-6-0s and later 4-6-2s to pull the train in the 1910’s-20s. But the all-heavyweight Empire Builder used 4-8-2 and 4-8-4 engines fro