Do not hump! - Trains Magazine
What can and cannot be humped is generally determined by the individual railroad, or even by the management at the specific yard. There are some cars, such as the glass-lined tank cars for refrigerated ethylene, that cannot be humped, loaded or empty. There are also regulations (Federal, I believe) about not humping (or kicking) cars in certain categories, depending on the color or design of the placard and the type of car or container on which it's applied.
At my local humping operation, we do not hump flat car loads of large combines, automobiles (wink, wink), or shiftable loads of steel (again, wink). Hazardous loads are given extra time for proper handling. Like Ed said in Mookie's thread about LP gas, we hump these "bombs" all the time, and there's very little to worry about. There are other types of cars, far less hazardous, that I'd rather refrain from humping because of what they can do to a yard or a track.
A lot is up to the individuals doing the work. Just because some of the restrictions on paper are often winked at, it doesn't mean that the cars are handled carelessly. Our track record is pretty good overall, and we do well when compared to other yards (which are supposedly far more sophisticated) on our railroad. And we don't have one poor railroad's record...a couple of weeks ago I heard from a source that should be reliable that General Motors (the number one customer for many railroads) will not give any business to one particular railroad because of damages incurred in its yard(s). Things have supposedly changed on this railroad, but GM has a long memory, and probably won't ship via this line for a long, long time.
Carl
Railroader Emeritus (practiced railroading for 46 years--and in 2010 I finally got it right!)
CAACSCOCOM--I don't want to behave improperly, so I just won't behave at all. (SM)