Local news outlets Efsyn, a Greek daily newspaper, and website Zougla – both of which received a call – said the explosive device had apparently been placed in a padlocked backpack on a scooter without licence plates.
A police bomb disposal squad arrived too late to safely detonate the device before it exploded, they said.
Dimas said in a statement: "This is a criminal act, which endangered the lives of people, employees and passers-by, in a central point of Athens and during peak traffic hour."
He added: "Nothing justifies terrorism, no act of violence brings justice. The authorities and the judiciary now have the floor."
The explosion occurred close to one of the Greek capital's busiest highways, Leoforos Andrea Siggrou.
Hellenic Train confirmed no employees or passing citizens were injured and that the blast caused "limited material damage".
It said: "Our company unequivocally condemns all forms of violence and tensions that fuel a climate of toxicity that undermines all progress."
In February 2023, a freight train and a passenger train carrying 350 people travelling in opposite directions were accidentally put on the same track. Most of the people killed were young students, while dozens were injured.
Multiple protests have been held in Greece since, including earlier this year to mark the crash's second anniversary.
Those demonstrations descended into violence, with hooded protesters seen throwing rocks and petrol bombs at police. Officers responded with tear gas and water cannons.
An inquiry concluded in February that the train crash was caused by human error, poor maintenance and inadequate staffing.
A date for a trial is yet to be announced.
