According to Frikret Zaman, deputy governor of the Bingol province, seven cars of a passenger train were derailed, killing the guards, and not Turkish troops, as earlier reported.
A second train that travelled to the scene to provide assistance was fired on by insurgents armed with rifles, an official said. It was not known whether there were any casualties in that attack.
The state-run Anatolian news agency said a postal train was targeted in the first attack.
Turkish troops found and defused a third bomb, placed on the tracks 200m away from the site of the first attacks, the Anatolia news agency said.
Twelve people, some of whom were trapped under the derailed cars of the first train, were injured in the attack, the agency said.
Paramilitiary forces launched an operation to hunt down the assailants, the agency added.
PKK fighters called off a
Local officials blamed Kurdish fighters for the attack.
One official said rebels from the separatist Kurdistan Workers Party (PKK) were believed to have laid C-4 plastic explosives on the track and triggered them by remote control.
The blast toppled carriages, and work had begun to rescue people trapped inside, an official said.
All 45 passenger and 11 crew members were evacuated by Turkish troops.
More than 30,000 people have been killed in the PKK’s armed campaign for self-rule in southeast Turkey since the group took up arms in 1984.
The clashes tailed off after PKK leader Abdullah Ocalan was captured and jailed in 1999, but there has been a resurgence of violence since the group called off a unilateral ceasefire last year.