As a follow-up to my earlier post on this topic, here are a few prototype examples (Yahoo willing) of chalk markings. Note that these were generated by railroad employees and are not graffiti.
The first markings applied to the sides of railroad equipment were made by railroad employees to communicate among themselves, typically in freight yards. These marks generally weren't applied as a way to say "I was here," but rather to convey important information that other railroad employees needed to know in order to perform their work.
Chalk marks were made using white and yellow chalk, charcoal, and lumber yard grease pencils. Chalk came in very large sticks, 1" in diameter and 4" long, as compared to normal chalk which was usually 3/8" by 3". These pieces of chalk were sometimes placed in a holder. The holder itself came from another essential railway tool, the discarded top off a fusee.
Most marking was done by yard clerks or “car markers.” They got their information from waybills. The marks were read by yard crews and freight train conductors. A waybill always accompanied a freight car, but if a car should go astray, or the waybill became lost, often it was possible to locate the car without much delay by the markings on its side.
Markings could communicate a variety of instructions and freight car conditions:
"MT" for empty.
"OK" or a check mark chalked over a truck to indicate a bearing been inspected
Assigned car spot
Condition of equipment that needs repair
Indication of an item's dimensions
Industrial sidings designations
Notations about routings and interchanges
Outgoing train numbers
Special handling or loading and unloading instructions
Whether the shipment was all going to several consignees and where it should be opened first
The codes varied from yard to yard and railroad to railroad and likely era to era. Today many of these would be impossible to decode, so simply copying what you see in prototype photos is probably the best way to model these. Chalking cars mostly died out in the 1980s.