Talked to the project head on our caboose, Central Vermont Railway 4052 which was built in 1923. The Sub-Floor was 1.75" Pine, couldn't figure out if it was White or Southern Yellow, and the walking surface was 3/4" Douglas Fir.

The replacement sub-floor boards we used are 11" x 1.5" pine, so they were probably standard 2x12 before the tongue and groove was cut. The cork layer was 1/2", so for the restoration due to the cost of cork panels we are using 3/4" plywood to make up the 1/4" difference.

Is only your floor sills steel, or do you have a steel superstructure in the body like our caboose has? if so, you might want to check the webs of the C-Channel side sills, due to trapped water in the bottom board we had to replace the web on both sides because they had become iron lace.

Take care and good luck with your restoration.

Regards,
Rich C.