Firstly, both articles need to get their chickens and eggs straight. What caused the trains to derail? Did exploding oil tanker cars cause the derailment or did the derailments cause the oil tanker cars to explode?
My other objections are with the folks cited in the articles. I'm all for safety but I smell some regulatory folks looking for another club to beat the oil industry. In addition, the comments along the lines of "crude oil doesnt explode like that" are gross over-generalizations. The ignition point (auto- or otherwise) will depend strongly upon the chemical components of the crude and crude compositions are all over the map from tar to what is effectively gasoline. Most of us grown-ups know one of those will ignite a bit more easily and spectacularly than the other.
I've always thought transport of crude oil by train was inherently more dangerous than by pipeline but then i guess the question of whether dangerous to cranes or dangerous to humans is the concern.
If there really is some merit to the supposition that Bakken oils are behaving differently from other rail transported oils (as opposed to there just being a lot more rail transported Bakken oil than other oils being transported by rail), then I doubt it is due to stimulating acids. If the oils are heavily emulsified when they go into the cars, then I wouldnt be surprised if a few hundred miles of jostling down a railroad track helped to separate an acidic aqueous phase which didn't play well with the steel but I suspect this would take repeated instances. If the cars are cleaned and inspected between loads, then I'd expect this problem would've been identified if it existed.
In addition, acid is used to stimulate many reservoirs and some of those oils must be transported by rail so if acid is the culprit, then there has to be something specific about the chemistry of the Bakken oil which makes the emulsion particularly unstable. Of course, I have no idea what kind of treatment the produced oil undergoes between the wellhead and the tanker cars. If the oil undergoes thorough de-watering prior to going into the cars, then corrosion by stimulating acid is almost certainly not the problem. Corrosion may still be a problem but it won't be due to those nefarious, evil fracking/stimulating fluids (and I get really sick of people who worry about c's or k's in fracking).
I wouldn't be surprised if the sustained mechanical vibration of rail transport caused some separation of crude components - either by destabilization of a dispersion or by destabilization of a kinetically inhibited metastable state. It's also possible that the ambient temperature may play a role - possibly in junction with the phase state (i.e. metastable solution or multi-phase dispersion).
However, I think it is much more likely that this 'story' is yet another case of folks not doing a proper statistical analysis which is more your bally-hoo. However, that analysis is frought with problems which is probably why folks are chasing other rabbits. I suspect the only other country where oil (with similar characteristics) is transported by rail in similar quantities to the Bakken is Russia and my prejudices would be that Russian rail lines and documentation of accidents is not quite up to par with those in the US and Canada.
ps: i'm back at my former employer but now north o the border
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