In Panjab,India one village called Pandiththa where due to excess usage of agrochemicals the soil is degraded and changed into poisonous one.  The farmers there used chemicals (in order to increase the wheat ) like Organo Phospates, Organo Chloride etc.  Instead of emiting Oxygen the trees in thise area emits poisonous air during photo synthesis.  So most of the people in that area are affected by the harmful diseases like cancer.  They are transferred from their place to the other places for the treatment.  So the trains in those areas are called as cancer express.

Courtesy –  from the Article - Dinamani, the Tamil magazine