John Sykes wrote:

That is one type of "Blue Flag" protection. Take a look here:
http://www.aldonco.com
-or-
http://www.aldonco.com/products.aspx?sec=2&cat=114&subid=20
Lots of fun toys for the 12" to the foot railroad enthusiast.

Might be worth pointing out that these images are of OSHA-standard signs, long postdating this list. Before 1960, signs were different -- and not standard.

Tony Thompson Editor, Signature Press, Berkeley, CA
2906 Forest Ave., Berkeley, CA 94705 www.signaturepress.com
(510) 540-6538; fax, (510) 540-1937; e-mail, tony@signaturepress.com
Publishers of books on railroad history