If you had a specific car series in mind, I can likely find a photo. In general though (if I recall correctly), CN's automobile boxcars had fishbelly centre sills, likely to provide extra strength to compensate for the gap in the body framing to accommodate the wider door opening. CN was known to retrofit power brakes to older cars, but I don't think this program was widespread – especially not prior to most of the earlier (and smaller) automobile cars being rebuilt to conventional boxcars beginning in the mid-1930s with removal of the auxiliary door.

Ian Cranstone
Osgoode, Ontario, Canada
lamontc@...