Modeling Santa Fe Passenger Cars
Santa Fe sill frame.

Otto Perry photo, Denver Public Library
The Santa Fe sill is a "C" shaped frame on all heavyweight cars built for the ATSF. The only exception you will see are Pullman sleepers and the fishbelly baggage cars. This sill is 3.5" deep and 9.5" high. I have used my wallet in the attached photo to illustrate.

For my modeling, I use Plastruct CFS-4 1/8" HIPS channel to simulate the sill. I lay the sill flat on a cutting surface and remove one side, leaving an "L" shaped piece which I glue to the bottom of my cars.