Ammonium nitrate is just the oxidizing element. It needs fuel. Standard "gunpowder" employs powdered charcoal. The OKC bombers mixed the ammonium nitrate with diesel fuel oil. Normally the fuel and oxidizer are homogenously combined for optimal results. It is hard to believe that sufficient amounts of separately shipped reagents on individual rail cars mixed correctly by "accident". I suspect high explosives (C4/Semtex/TNT) were purposely loaded on a rail car for the utilitarian purpose of eliminating Kim Jong Il.
The coal cars for my project in Birmingham carry 110 tons of coal each. Replace that with TNT and you get a yield close to what has been reported.