Workmen digging a hole above a Moscow underground train line drove a concrete pillar into the roof of a moving train today.
A spokesperson for Russia's Emergencies Ministry said a fire broke out after the impact.
500 people were evacuated from the underground and no one was hurt.
Prosecutors said they had begun a criminal investigation into suspected violations of the building safety code, the Interfax news agency reported.
The accident took place on the Zamoskvoretskaya line, about six kilometres north of Moscow's city centre.
Train services on the line were suspended. Services are expected to be back to normal by tomorrow morning.
