UPDATE: AP ups the casualty count to 20, and expected to rise. Michelle has more, and points out that it's 7/11.
Dozens of people were injured in the blasts, which took place around 6:30 p.m. (9 a.m. ET) when the trains were packed with commuters making their way home.
A correspondent for CNN's sister network, CNN-IBN, reported seeing 15 bodies at the Matunga train station. Video from one station showed people with blood on them being treated, other commuters carrying victims and some people lying motionless near train tracks.
At least one train was split in half by the explosion.
Mumbai's subway system was put on red alert following the blasts on trains in the city's western suburbs, and police in the capital New Delhi also heightened security.
Airports across India were also put on high alert, and Prime Minister Dr. Manmohan Singh called an emergency meeting of his ministers after the explosions.
Fox News reports the attack has "all the hallmarks of an al Qaeda attack, but no claims of responsibility yet."
More from AP:
The first explosion hit the train at a railway station in the northwestern suburb of Khar, said a police officer who spoke on the condition of anonymity because he was not authorized to talk to the media.
India's CNN-IBN television news, which had a reporter traveling on the train, said the blast took place in a first-class car as the train was moving, ripping through the compartment and killing more than a dozen people.