Tim and Mike;
The diagram in my 1960 copy indicates that it is for both flats and gons
("RAILS - FLAT OR GONDOLA CARS").

The rail is laid in rows on top of 4 - 1x3"s the full width of the car.
On flats, the sides are secured by hardwood stakes, extending at least
3" above the top of the load. At least 3 - 2" high tension bands would
be wrapped around the load, with the end ones approx 7' from the ends;
but, alternatively, you could also use hardwood side and top restraints,
bolted through the floor. Each layer was separated by at least 2 strips
of 1 x 3".

The only place the end protection is cited is in use with rail less than
12 ft long, where boards 2 in thick are nailed to the inside of the
stakes.

That being said, I saw many more loads back when secured according to
the following note at the end, which states, "Material need not be
secured when loaded below the top sides of gondola cars, with end gates
raised and fastened..." They did use dunnage under the load, though.

I have a P&LE USRA gon with this type of loose load (actually small
channel) that I just finished recently. Very cool-looking.

Elden Gatwood