On Thu, Dec 3, 2020 at 04:25 PM, Tony Thompson wrote:

A man who had worked for the SP once said to  me that he didn't know the marks used in yards other than his own, and didn't always know the marks used by the guys on swing shift.

I've done some research on chalk markings and this is the same information that I have found. The markings varied by yard, crew, placement and years. Sometimes the marks are intuitive but often not. They have become somewhat of a dead language.
Bob Chaparro
Hemet, CA