The train carrying tourists to Portugal veered off the tracks and smashed into a pillar on Friday, killing its Portuguese driver as well as a US passenger and two Spaniards.
The train's so-called black box recorder was recovered and handed over to judicial authorities in the small town of O Porrino where the tragedy occurred. "We are reviewing the possible causes of the accident and the most probable is excessive speed," O Porrino mayor Eva García told reporters.
She cautioned however that definitive conclusions would only be made public when an investigation was finished, at a date to be determined.
The train, which was travelling to Porto in Portugal from Spain's Vigo, appeared to have hit the wall of a bridge as it was going underneath, prompting it to crash just before entering a station.
Authorities in the Galicia region where the accident took place said that the train conductor was one of the two Spaniards who died in the Friday morning accident, which also saw some 47 people injured.
The train itself had undergone a complete overhaul in May, Spanish officials said, while Portugal's rail chief said it was "in perfect condition".
Galicia was also the scene of one of Spain's worst rail disasters in 2013, when around 80 people were killed and another 144 injured after a high-speed train slammed into a concrete wall on the outskirts of Santiago de Compostela.
That train was approaching a curve at more than twice the speed limit on that piece of the track.