SoDo Sign Zen: No Hammers

The parking lot of the SoDo Home Depot abuts a railway track — perhaps a spur line or a storage area for the more active line that occasionally cuts off east-west traffic between Occidental Ave S and 3rd South. On one of the boxcars stored there was the curious admonition pictured above. No text, just the imperative against hammers. Any ideas what it could mean?



Also spotted on a train car carrying grain near a flour mill on Harbor Island:
http://buffoonery.org/oldphotolog/imageview.php?date=02.25.03&number=11
Maybe it has to do with the flammable gases that can build up in a container of grain. Hammers make sparks?
I think you’re right about the sparks, because I saw a text version of the warning on another railcar north of downtown today. I don’t remember exactly what it said, but it was definitely something about flammable contents and it specified that one shouldn’t hammer on that car.
Now I wonder if someone could tell me what the “no humping” I saw written on another car might mean? It gave me some delightful mental images.