The blast struck the train as it pulled out of the spa town of Yessentuki, some 550 miles south of Moscow, during the morning rush hour and many of the victims were said to be students, the Xinhuanet news agency reported Friday.
The force of the bomb, which one official estimated at more than 20 pounds of plastic explosives, not only killed 42, but hurled bodies and body parts dozens of yards, the Russian Emergency Situations Ministry announced.
Many of the wounded were students on their way to schools in the resort city of Mineralniye Vodi.
The director of the Federal Security Service, Nikolai Patrushev, said a male suicide attacker and three women accomplices appeared to have carried out the attack.
Two of the female attackers leapt from the train seconds before the explosion while the third woman was seriously injured and unlikely to survive.
Patrushev said hand grenades attached to the legs of the male suspect indicated he had been a suicide attacker.
No claims of responsibility for the blast were reported in the immediate aftermath.
A Russian minister said the attack bore the hallmarks of Chechen rebels.