I believe in the 1920s the Rio Grande Southern used a Ford Model T as the base for an inspection car.
When the Depression hit small and short lines, one of the ways to economize was to scale back the size of the train. If you had the mail and express contracts, you still wanted to run the same schedule as before. Many shop crews realized that it was much less expensive to run a converted automobile than to keep one of the engines in steam.
So a lot of model T’s, as well as other autos were converted into railcars and put to work hauling mail, express, and a few passengers. As well, there were other model T’s which were made into inspection cars, or trackworker’s vehicles, sometimes pulling a trailer with tools and supplies, sometimes not.
Later on, manufacturers began to produce railcars and railbuses, using some of the same techniques that the rairoad shop personnel had invented.
Some of the very first railcars produced by railroad shop workers were pretty bizarre looking.
-Ed