From the NRA page:
and if you look at 49CFR173.54, it prohibits loaded firearms on interstate commercial transportation, because they are a prohibited explosive:
FIREARMS ABOARD OTHER CARRIERS
Any passenger who owns or legally possesses a firearm being transported aboard any common or contract carrier for movement with the passenger in interstate or foreign commerce must deliver the unloaded firearm into the custody of the pilot, captain, conductor, or operator of such common or contract carrier for the duration of the trip. Check with each carrier before your trip to avoid problems.
Bus companies generally refuse to transport firearms. Amtrack specifically prohibits the transport or possession of firearms and ammunition both on board and in checked baggage. Other passenger rail companies may also restrict transport or possession; always check with your carrier first.
and if you look at 49CFR173.54, it prohibits loaded firearms on interstate commercial transportation, because they are a prohibited explosive:
TITLE 49--TRANSPORTATION
CHAPTER I--PIPELINE AND HAZARDOUS MATERIALS SAFETY ADMINISTRATION,
DEPARTMENT OF TRANSPORTATION
PART 173_SHIPPERS_GENERAL REQUIREMENTS FOR SHIPMENTS AND PACKAGINGS--
Subpart C_Definitions, Classification and Packaging for Class 1
Sec. 173.54 Forbidden explosives.
Unless otherwise provided in this subchapter, the following
explosives shall not be offered for transportation or transported:
(snip out irrelevant parts)
(f) A loaded firearm (except as provided in 49 CFR 1544.219)