Windows were broken within six miles of the blast.
I rather doubt that this would be the result of an explosion resulting from accidental mixtures of fuel oil and fertilizer...
The mixture itself is believable, since that's what was used in Oklahoma City. What isn't believable is that these elements just happened to combine on a moving train in just the right proportions to form a high explosive, AND THEN managed to get itself detonated (something which ordinarily needs an explosive such as a blasting cap). Unless they decided to save space by mixing the fertilizer, fuel oil, and blasting caps together while they transported them, I'm not buying this explanation.
The railroad system in Iran was originally built to western standards, but has fallen into sad disrepair under the Mullahs.
Sorry gentlemen, but the Iranian train wreck represents nothing more than folks with 7th century industrial skills trying to operate 21st century mechanical equipment.
Allah must be very proud.