In Istanbul according to reports
Bloomberg say it's at Bayrampasa station
Reuters now picking up more saying trains we're halted after a "loud noise" and local media are reporting it as an explosion
Hopefully it's nothing more than over-imaginative reporting
Update: Seems not as BBG is now saying that 1 person has been killed and 1 injured, citing HT
Update 2: Explosion is believed to have been caused by a power transformer blowing up, according to HaberTurk