I propose the other way.
They should be sheathed so they look like 1920's cars to fit
the Museum's time period.
If the Railroad portrays what it would be if it had continued until the 2020's
then ALL of the old equipment would be gone - Vacuum Brakes, Archbar Trucks,
Wood Frames, & Steam Engines.
So lets keep the rolling stock as period appropriate as possible.
I know there are late 20th Century intrusions but they should be kept to a minimum,
and behind the Green Curtain (With the Wizard
)