"I wonder why they have to mark a car especially for this? My guess was that it was for switching practice of some kind."
Let me have a turn at explaining, too: railroading is a complicated business; these are classroom cars, used to train employees (typically operating, MOW and maintenance - of various kinds-; others, certainly) hoe to do their jobs.
There is a good photo - can't find it right now - taken inside a Gulf, Mobile & Ohio classroom car set up in the 40's to train enginemen in how to operate diesel-electric locos as the GM&O was transitioning from steam. Large scale model, cut-away machinery and drawings, etc. It was converted from an old passenger coach.
These cars are schools. Railroads will bite you in two - literally - if you don't know what you are doing.