The young man from the Andalusian town of Andújar suffered the fatal blow when he touched a high-voltage wire on a train he and his friends had assumed was not electrified.
They'd clambered on top of the parked wagon at Andújar train station to take a picture of themselves when they suffered the severe electric shock.
Spanish online daily Ideal reported that a 3,5000-volt discharge killed the young man as he went to take the photo.
Local paramedics and police rushed to the scene but found the 21-year-old was already dead.
One of his friends survived the shock but remains in a serious condition at the local Alto Guadalquivir Hospital.
Andújar's mayor has declared three days of mourning for the town of 40,000 people in the middle of olive growing country.