By Sanjoy Majumder
BBC News, Mumbai
Mumbai is no stranger to violence.
This is the first attack on Mumbai's railway network
In August 2003, more than 55 people died in twin bomb blasts in the city's financial district.
And in 1993, some 250 people died and nearly 1,000 were injured in a series of bomb blasts which rocked the city.
Both attacks were said to have carried out by Islamic militants allegedly at the behest of Mumbai's criminal underworld.
They were believed to be reprisal attacks in response to religious violence elsewhere in India in which Muslims had been targeted.
It is still not clear who is behind these latest bombings and certainly investigators will be puzzled over the motive.
Earlier on Tuesday, the summer capital of Indian-administered Kashmir witnessed a series of grenade attacks carried out by suspected separatist militants.
The victims of the attacks cut across the city's ethnic, religious and class lines affecting both blue and white collar workers
It is unlikely that the two are related although India's security agencies will certainly look for any links.
But perhaps the closest related event took place last year, when the Indian capital Delhi was also rocked by a series of blasts.
As in Mumbai, it was ordinary Indians who were targeted in a series of blasts which took place in congested markets and shopping areas where the impact was the greatest.
Mumbai is India's commercial capital and its rail network is often described as the city's lifeline.
Two major lines cut through the city, running north to south, bringing in commuters from distant suburbs.
An attack on the rail network does not merely affect a large number of people, it is also designed to bring the city to a halt.
Security has been tightened across the country
Tuesday's blasts took place on the city's Western Line which connects the city centre with some of the more affluent suburbs.
The victims of the attacks cut across the city's ethnic, religious and class lines affecting both blue and white collar workers.
Even as investigators piece through the wreckage at the bomb sites, searching for leads, it is quite evident that the explosives used were both sophisticated and powerful.
And the co-ordinated nature of the blasts, occurring in near succession, speak of a degree of organisation that few can command.
Security agencies and investigators will quite likely look for any links to some of the militant groups that have been accused of carrying out similar attacks in the past.
That includes both groups fighting Indian rule in the disputed state of Kashmir as well as militant groups with ties to Mumbai's organised crime syndicates that have been implicated in previous attacks in the city.
In the past, Indian officials have been quick to accuse Pakistan of supporting or providing a safe haven to several anti-Indian groups.
Attacks such as the one on India's parliament in December 2001 led swiftly to a war of words between the two South Asian rivals.
But a peace process has been under way for more than two years and Islamabad has been quick to condemn the blasts, something that will be welcomed in India.
For the moment, the main challenge confronting the authorities is to dispel any sense of panic among the city's residents.
In particular, they will hope to contain any strong reaction which could upset the city's delicate religious balance.