At least 36 people died and many others were hurt when two trains crashed into each other late Tuesday in northern Greece.
One train was carrying people and the other was moving goods.
Many of those who died were young people in their 20s.
The accident took place near the Vale of Tempe, about 380 kilometers north of Athens.
Investigators are working to find the cause of the crash.
Greek police said they arrested the stationmaster in the nearby city of Larissa.
State broadcaster ERT said the trains were traveling at speeds of more than 140 kilometers per hour when they ran into each other.
Survivors said the crash threw passengers through windows.
Some bodies were found 40 meters from the crash site.
Stefanos Gogakos was sitting near the back of the passenger train.
He said it felt like an explosion.
He could see a fire at the front of the train.
He was covered in glass, he said.
"My head hit the roof," he said.
"Some people started to climb out through the windows because there was smoke in the carriage."
On Wednesday, pictures showed several train cars off the tracks.
Some of the train cars were on top of each other.
Emergency trucks were all around the area.
A crane was also there to lift the train cars off one another.
Greek Prime Minister Kyriakos Mitsotakis visited the crash site.
He said, "I can guarantee one thing: We will find out the causes of this tragedy and we will do all that's in our power so that something like this never happens again."
Greek President Katerina Sakellaropoulou went to the site to put flowers down.
The government said there would be three days of national mourning and flags flew at half-staff outside the European Commission in Brussels.
Greece's transport minister, Kostas Karamanlis, resigned on Wednesday.
He said he was taking responsibility for the country's "long-standing failures" to fix a railway system that he said was not fit for the 21st century.
Rescue worker Lazaros Sarianidis told ERT "it will take a long time" to move the train cars and remove the bodies.
Vassilis Polyzos lives near the crash site.
He was one of the first people to arrive at the scene.
"The trains were completely destroyed," he said. "People ... were scared – very scared."
The Greek Railroad Workers Union said the drivers of the two trains were both killed.
Greece's firefighting service said 66 people were taken to hospitals.
More than 200 people who survived the crash without serious injuries were taken by bus to Thessaloniki, about 130 kilometers away.
I'm Dan Friedell.