People who use the Tube and trains are most at risk of becoming terrorist attack victims than those who fly, according to a study at MIT.
Arnold Barnett, a professor of management science at the Massachusetts Institute of Technology, rail users are more likely to fall victim to attacks than frequent fliers or people engaged in "virtually any other activity".
In July 2005, a double decker bus and trains on the London Underground were attacked by suicide bombers. In total, 52 people were killed and over 700 more were injured.
A year earlier, a series of bombs went off on commuter trains in Madrid, killing 191 people and wounding over 1,800.
The study found that terrorist attacks against aircraft have dropped significantly, while those on trains and subways have surged.
According to Barnett's paper, Has Terror Gone to Ground?, between 1982 and 1991, 1,327 people across the world died from deliberate acts of malice during air travel. During this time, there were no deaths from malice on subways and commuter trains.
From 2002-11, however, 203 people died from malicious acts on aircrafts while there were 804 malicious deaths on trains. Measures to improve security on aircraft may be the reason there have been fewer attacks, the report suggests.
Barnett notes that while rail travel is more risky than air travel, the chance of being killed in a terrorist attack on a train is still very low - around 1 in two million per year.
Even so, he concludes that the prevention of rail terrorism should be a priority and urges better intelligence to uncover plots at early stages.
Barnett's paper follows claims made by the Russian security chief, who said terrorists have changed their tactics. Alexander Bortnikov said terrorists were setting fire to European forests in a method dubbed "forest jihad".
He said this method of terrorism is effective because it requires little training, is difficult for police to arrest perpetrators and it inflicts moral and physical damage.