PROY-NOM-021-SCT2/2015 "Disposiciones de compatibilidad y segregación en trenes de unidades de arrastre que transportan materiales y residuos peligrosos" 

(Draft Mexican Official Standard PROY-NOM-021-SCT2/2015: Provisions concerning compatibility and segregation in trains composed of rail haulage units transporting hazardous materials and waste) (9 pages, in Spanish)

One of the main problems that may arise in the event of a train derailment is the spillage of hazardous materials and waste. The substances involved may be liquids, solids or gases, and be volatile or reactive. This means that certain types of substances may, when leaking from the area in which they are contained, react with oxygen in the air or any other chemical element present nearby. Such reactions can be violent (explosion) or non-violent, but risk polluting the surrounding land, water or air. They may, in extreme cases, lead to the death of living organisms.

Hazardous materials are classified by the United Nations as explosives (substances producing a pyrotechnic effect), gases (liquefied, compressed or dissolved under pressure), liquids (including mixtures of liquids or liquids containing solids that can give off a flammable vapour at low temperatures), solids (substances liable to spontaneous combustion or which, on contact with water emit flammable gases), oxidizing substances, toxic substances, radioactive material, corrosive substances or miscellaneous dangerous substances. Since trains may be composed of units carrying different types of substances, care must be taken to ensure the appropriate train formation and segregation of substances depending on the contents of each wagon, so as to prevent any reaction between the substances concerned.