Thanks for your thoughtful comments. You must be quite knowledgeable about explosives.
Dynamite was the explanation given to aid workers by N. Korean officials when they were shown the site of the explosion a couple of days after the incident.
150 dead after rail blast: N Korea officials "the explosion had been caused by dynamite"
Next, it was a dynamite explosion due to contact with loose electrical wire(electric shock?)
Rail Blast: N.Korea Accepts UN Help
Later, it was back to ANFO explosion triggered by electrical charge from
N. Korea Cites Human Error in Crash
From these reports, It is possible that N. Koreans use dynamite and ammonium nitrate interchangeably, as you speculated. By the way, some Freepers voiced their doubts that electrical charge alone could detonate ANFO or dynamite whichever you call the explosive material. Do you think it is likely?
Correction:
Later, it was back to ANFO explosion triggered by electrical charge from -->
Later, it was back to ANFO explosion from contact with a loose electrical wire.
I wasn't suggesting an interchangeability, I was merely addressing the possibility that dynamite might also have been carried on the train. Normally, an ANFO blast uses a base charge of ammonium nitrate and fuel oil that is detonated by a booster charge (typically Pentolite 50) which in turn would be detonated by a #8 blasting cap. The blasting cap is typically set off with det cord. I'm just speculating here, but the North Koreans are poor and tend to run everything on a shoestring and would likely substitute the cheaper dynamite for the Pentolite booster.
Nevertheless, for safety reasons and security reasons (real important in a tyranny like NK), the boosters, blasting caps and det cord would likely be shipped separately, probably in an (Army) truck, and not on the train. (One truck, even a pickup truck, could easily carry the necessary detonation explosives for 80 metric tons of ammonium nitrate.)
In other words, I believe there was no dynamite on the train and that the use of the term "dynamite" in regards to the train contents, was a misnomer and was (improperly) used in reference to the ammonium nitrate.
•"...some Freepers voiced their doubts that electrical charge alone could detonate ANFO."
As long as we're not talking about a bolt of lightning, electrical discharge, by itself, will NOT set off an ANFO charge. While there are electrical blasting caps, they contain very sensitive primary explosive charges that can be detonated by an electrical discharge. ANFO is a very insensitive, secondary explosive.
--Boot Hill