In a message dated 5/10/05 8:27:41 PM Pacific Daylight Time,
<< I assume from the way you've put it that this was the creation of
placarding, and not just the mandating of a convenient method for changeable placards.
But is it possible that the innovation was the metal holders themselves? >>
Placarding was used prior to the ICC mandate, obvious within both builder
photos and in service shots. The mandate was submitted to ballot vote in 1931 and
the ARA membership voted unanimously to accept the provisions within.
All newly constructed tank cars as well as those receiving general repairs,
on or after, January 1, 1932 were required to be equipped with permanent metal
Placard Holders. Note that the provision (as approved) made no distinction
for cars transporting "dangerous" commodities.
Earthquake Faults, Nevada