The source, who spoke on condition of anonymity due to restrictions on talking to media, said the explosive had been planted on the railway track in the province's Sur district.
The source said the explosion caused no human casualties.
The wagons were all empty but several were derailed by the blast.
After the attack, an air-backed operation was launched in the region.
The PKK, listed as a terrorist organization by Turkey, the U.S., and EU -- resumed its armed campaign in July 2015 and since then has been responsible for the deaths of approximately 1,100 security personnel and civilians, including women and children.