Investigators are working the scene to determine a cause. One train apparently rammed a stationary train on the system's Red Line. One of the dead was identified as the operator of the moving train.
The Washington Post cited signal system failure and operator error as possible causes of the accident.
The force of the impact crushed the rear car of the sitting train "like an accordion," witnesses said, and forced the moving train into the air. It landed atop the other train.
Rescue crews searched the wreckage and surrounding area for victims. The Post said five bodies were removed from the crash debris Tuesday morning.
Commuters Tuesday were warned to avoid using the Red Line if possible. The Red Line is the busiest section of Washington's Metro system.
Iranian Supreme Leader Ayatollah Ali Khamenei in his Friday sermon called for an end to street protests that have grabbed Iran since the results of last week's presidential election were announced.
Khamenei said the demonstrations were a "challenge" to democracy and asked how could anyone rig an election that one candidate won by 11 million votes. That candidate would be Iranian President Mahmoud Ahmadinejad.
"The Islamic republic state would not cheat and would not betray the vote of the people," Khamenei said.
The ayatollah also warned that people who urge protests would be held responsible for any resulting "chaos."
Iranian protesters, clad in black for mourning, filled Tehran's streets in a post-election rally Thursday that began and ended peacefully, witnesses said.
Tens of thousands of protesters wore black in remembrance of those who died in the violence that followed Friday's elections in which President Mahmoud Ahmadinejad was declared the landslide winner over his nearest challenger, Mir Hossein Mousavi.
Mousavi addressed the throng during the "ceremony of mourning," messages posted on Twitter indicated. The Iranian government banned foreign media from live coverage of "unauthorized" demonstrations, so accounts are based on Internet postings by witnesses or participants.