From my horse raising days, I can tell you that thick white oak planks are pretty tough stuff and stand up to a lot of horse wear and urine/manure issues. If you ever go into a professionally built horse stable built in the 1960s or 70s, you will still see lots of it.
And, eventually, after many years of wear and tear and abuse, you just replace it.
Horse and cattle trailers use to have their metal floors lined with it. (And the owners just hosed out the trailers after they was used.)
I am not sure how the railroads protected the underlying metal in the rail cars, but perhaps they just hosed the cars out as well.
I can tell you that two layers of 30# tar paper is darned near indestructible as far as liquids, so perhaps they put down something like this underneath the planks.
Horses and cattle can slip and fall pretty easily on wet metal surfaces, so it would be hard to understand the rail cars just having them stand on metal floors.
Mannyrock
