Tin-plated metal is typically a rust-proofing finish in leiu of paint, appropriate for rails and wheels, although it tends to wear off. I'm thinking the "tinplate" term actually refers to rails and wheels more than the actual train cars, which generally had painted bodies. Paint would adher better to bare metal than to tinplated metal. Also, paint or enamel is a more attractive and economical finish for most consumer products.
Tin-plate metal is most commonly used for cans. Prewar trains were largely sheet steel for the shells, so maybe the idea just carried over that the train cars were like tin cans on wheels, even though the metal was generally painted. What's your take on it?
The tin protects the steel against rusting and corrosion and also acts as a lubricant when the steel passes between the surfaces of dies during the shaping of the cans.
So if tin was a 'lubricant' for die work, this would have helped with shaping tubular rails and pressed-steel wheels.
I have these old #607-607-608 Lionel passenger cars from the 1920's which have lost their paint. The cars show evidence of tin plate on the bodies, which seems odd to me. Why use more expensive tin-plate metal if it's painted over? The paint wouldn't adhere as well.
