Is there an equivalent of cash transportation in air cargo?
Decades ago, when we moved checks at night for Federal Reserve clearing, we would sometimes have sealed bags which were cash.  On my flights, I never had an armed guard on board, but on taking off and landing, we were met on the ramp by an armored truck with armed guards on a routine basis.  

The impression I had is that checks and other paper were the main cargo, and that cash and bullion were minor incidentals.  

Related, items of national security are routinely carried by scheduled airliners, and are placed into the hold by cleared individuals, and then the hold is opened upon arrival, in the presence of cleared individuals who retrieve the packages.  Contents are logged, in the event of compromise.  Similar parcels are also handled by the major parcel and courier enterprises, with slightly different processes.