Where does the article say that the bomb was going to be detonated on a train? Or that it would have been carried out by a suicide bomber with a small rucksack? It doesn't.  What it actually says, is:  Had the fifth bomb (which was going to be planted in the most vulnerable place, rather than exploded in a moving train) succeeded in breaking the tunnel roof, the resulting torrent of water and shorting out of the electricity supply would have knocked out even the pumps that keep the normal leaks under control, thus making things even worse. It would take a mass of explosives for it to work, it simply does not add up.