Ah, so, my guess was pretty close to being right.
My uncle worked for the Lehigh Valley RR as a carpenter building boxcars, etc. and I can remember going with him as a child to the yards and seeing chalked markings on the side of cars. I was intrigued because it had never occurred to me that chalk could be used to write on anything besides slate blackboards.
Thanks for the explanation.
Sorry for the OT diversion, Tim.