Russian authorities say the blast from a bomb planted on train tracks in the southern Dagestan republic has killed one person and wounded four others.
The commuter train was heading from a town in western Dagestan, Khasavyurt, to the regional capital, Makhachkala, early Sunday when the explosion occurred.
Russian media say police believe the bomb was detonated by remote control.
Dagestan authorities are calling the bombing an act of terrorism, and the Russian news agency Itar-Tass quotes officials as saying the attack was aimed at harming civilians.
There is no immediate claim of responsibility, but police say they suspect Muslim extremists carried out the bombing.
A new Russian study found that violence by Muslim separatist militants from Chechnya is spilling over to neighboring Caucasus republics such as Dagestan, where the report says terrorist attacks have more than doubled to 70 since last year.
Some information for this report provided by AP and AFP.