According to Fred W. Frailey’s Twilight of the Great Trains, 1998, Kalmbach, during the 1960’s, the Santa Fe “applied silver paint to underbodies of cars and diesels before every backup move into Dearborn Station.” (p. 59).
First, in railroad terminology, what is an underbody? The wheel trucks? To my mind, an underbody is not likely to be seen by any passenger or onlooker. It would be visible if you laid flat on your back as a train rolled over you. (Not a recommended position.)
Second, regardless of where it was applied, that is a lot of paint. Given that each trainset would make at least two roundtrips a week, that means a minimum of 100 coats of silver paint were applied each year. That could only result in a heavy buildup that would have to be sandblasted off, unless such sandblasting was also done in preparation of a new coat being applied each and every time.