Terrorists target vulnerable rail networks
Despite armed police patrolling stations across the world, experts have long feared that rail networks were particularly vulnerable to a terrorist attack. Between 2002 and 2011 there were 203 deaths on aircraft that were attributable to acts of malice. On trains and mass transit there were 804.
These included the commuter rail bombings in Madrid and Mumbai as well as the 7/7 attacks in London.
The most detailed research on the subject has been carried out by Prof Arnold Barnett, a statistician at MIT's Sloan School. He calculated that rail passengers were twice as likely to be victims of terrorism as frequent flyers.
The risk per mile when travelling by train was 10 times that for an air passenger.
In late 2013, citing intelligence sources, the German tabloid Bild reported that jihadists had targeted Europe's high-speed rail network.
It was believed that terrorists would plant bombs, possibly in tunnels or to try to destroy tracks or electric cabling.
High-speed rail has become increasingly important in Europe to link major capital cities and financial centres, with direct trains from London to Paris and Brussels, and a direct service to Amsterdam planned.
But one of the attractions of rail travel - its convenience - could be seen as its weakness. There are security checks, but they are light in comparison to the precautions taken at airports.
In the US, although air and road travel dominate, there is a flourishing inter-city network in some parts of the country, notably on the east coast.
According to official estimates, Americans make 3.5 billion train trips per year, with security tight in the shape of armed police and random searches of passengers. Bomb sniffer dogs are a common sight at major terminals such as New York's Penn Station.
Despite the greater precautions introduced, the US continued to spend $9 on aviation security for every $1 it spent on security for trains, a priority that has been questioned by some in congress.