I found information about chalk markings from a number of sources which I incorporated into a clinic. Below are some of what I found.
Markings could communicate a variety of instructions and freight car conditions:
Required handling of a specific car or load
Condition of equipment that needed repair
"OK" or a check mark chalked over a truck to indicate a bearing had been checked
Indication of an item's dimensions
Notations about car routings and interchanges
Track assignments for individual cars
Outgoing train numbers
Industrial siding information
Assigned car spot at a customer’s facility
Humping instructions for cars switched in a gravity yard
“MT” for an empty car
Whether the shipment was all going to one consignee or where it should be first delivered.
Special loading and unloading instructions
Chalk marks were not a universal language. Codes varied from yard to yard, railroad to railroad and era to era.
To determine which chalk marks to respond to, sometimes individual yard crewmen would make their mark in one particular area so fellow crewman would know how to respond to what might be a jumble of chalk marks. Or a crewman might take the time to wipe off a mark and this occasionally is seen in photos.
Here is how one railroader described his work:
“I read the destination cards stapled to the cars. We could determine where the various blocks started and ended and then chalk the cuts. The other switchman marked the cut positions using bold letters and arrows so we could find them in the dark.
Doing this was far better than standing next to the train looking at a list, trying to find a car by its number.”