The serial blasts on Mumbai's suburban trains that killed at least 174 people on Tuesday has proved again that trains, particularly during rush hour, are soft, enticing targets for terrorists.

One, because they cause maximum damage and panic, thereby serving the terrorists' purpose, and two, because preventing such attacks is next to impossible -- even with intelligence inputs -- without severely disrupting commuter traffic.

Given overpopulated Mumbai's heavy dependence on the train system, it is an obvious target.

Tuesday's attack was not the first such attack. And it certainly won't be the last.

A quick look through the past decade where trains have been attacked by terrorists.

In March 1995, members of the Japanese religious cult Aum Shinrikyo released sarin gas, a nerve agent, in the Tokyo underground during rush hour in March 1995, killing 12 and injuring thousands.

Barely four months later, in July, suspected Algerian Muslims used gas canister bombs inside the Paris Metro, killing eight and injuring 90 people.

Less than a year later, in June 1996, Chechen rebels were said to be behind an explosion on the Moscow [Images] underground, in which four people died and 12 were hurt badly.

In July, two massive explosions aboard a commuter train in Sri Lanka's [Images] capital Colombo, killed 60 people and injured nearly 500. The Liberation Tigers of Tamil Eelam are blamed for the attack.

And in December that year, a bomb planted in a packed train by Bodo extremists in the northeast Indian state of Assam, killed 300 people.

Scattered incidents took place over the next three years, and then in December 2000, at least 11 people died when a bomb exploded on a train in Manila, Philippines. 60 people were hurt.

A year later, in August 2001, a landmine planted by UNITA rebels on a rail track in Luanda, Angola, at least 252 passengers.

A year later, on February 18, 2003, a suspected arson attack killed some 300 people during rush hour in Taegu, South Korea.

Barely a month later, on March 13, 10 people died when a bomb planted by suspected extremists exploded aboard a commuter train in Mulund, north Mumbai.

February 2004 saw a woman suicide bomber self destructing in the Moscow underground, killing at least 40 commuters.

This was followed by the Madrid train blasts of March 11, in which 191 people died and hundreds were injured.

The next year, on July 7, 2005, four suicide bombers struck on London's underground railway network and on a double-decker bus, killing at least 60 people and injuring hundreds.

A similar attempt on July 21 failed due to faulty explosives.

Tuesday's blasts in Mumbai thus fits a grisly pattern of striking at unsuspecting commuters headed to or from work.