For CRS 18-9-118.
18-9-118. Firearms, explosives, or incendiary devices in facilities of public transportation

A person commits a class 6 felony if, without legal authority, he has any loaded firearm or explosive or incendiary device, as defined in section 9-7-103, C.R.S., in his possession in, or carries, brings, or causes to be carried or brought any of such items into, any facility of public transportation, as defined in section 18-9-115 (4).

section 9-7-10 - defines EXPLOSIVE and INCENDIARY DEVICE

Note: Excludes ammunition (or components) from being classified as an explosive.
(3) "Explosive" or "explosive device" means any material or container containing a chemical compound or mixture that is commonly used or intended for the purpose of producing an explosion and that contains any oxidizing and combustible materials or other ingredients in such proportions, quantities, or packing that an ignition by fire, by friction, by concussion, or by detonation of any part of the compound or mixture may cause such a sudden generation of highly heated gases that the resultant gaseous pressures are capable of producing destructive effects on contiguous objects, but shall not mean the components for handloading rifle, pistol, and shotgun ammunition and/or rifle, pistol, and shotgun ammunition.
Q: Can I carry ammo, powder, etc. (without having a firearm on one's person) onto Public Transportation ?
A: YES, because it is not defined as a prohibited/restricted item (i.e., an EXPLOSIVE).

Q: Can I carry a firearm (loaded or unloaded) onto Public Transportation ?
A: SOMETIMES. One needs to look at Colorado and Federal Statutes (and Denver for RTD)

For Colorado:
NO PROHIBITION, because a Colorado CCW permit is authorization from your County Sheriff, per Colorado, for you to carry a weapon. As such, you are responsible for complying with State and Federal statues. It is the "legal authority" derived from the CCW statutes that CRS 18-9-118 requires. "Legal Authority" does not mean "only law officers etc.", but extends to all those permitted by law.

For Federal:
SOME PROHIBITION - Federal law prohibits carry (on one's person) firearms for
... Commercial Airlines (exceptions made for certain types of LEOs) See 49 USC 46505 , 49 US 46303
... Amtrak Trains (Can't find any references yet ...)

Denver
Can't find any references yet