MOSCOW — Several carriages were derailed in the crash, which happened at 12:38 (08:38 GMT) near Bekasovo 1 station, 60km (37 miles) from the capital.
Injured people were reportedly carried from the train as emergency services rushed to the scene.
The passenger train was on its way from Moscow to Chisinau in Moldova.
Russia’s interior ministry said at least five people were killed and 15 were injured, although some local reports put the number of injured at as many as 45.
Officials said several carriages on the freight train came off the rails near the town of Naro-Fominsk and hit the passenger train, which was reportedly carrying about 400 people.
Rescue coordinator Vadim Andronov told Russian news agency Itar-Tass that the death toll was likely to rise.
“One of the carriages of the passenger train was crushed by the freight train wagons,” he said.
“Rescuers are working to pull out injured people being crushed by the wagon.”
A spokesman for the health ministry, Oleg Salagay, said that rescue crews and medical teams were doing everything they could to save lives.
“Medics are working at the scene now, assessing the condition of the injured,” he said.
“All the necessary medical aid is being provided for them, and this gives us hope for a successful outcome.”
The cause of the accident was not immediately clear.
Traffic on the line – which also serves Kiev in Ukraine – was suspended as a result.
Source: BBC News