A policeman guards the site of the collision of two subway trains in Mexico City, capital of Mexico, May 4, 2015. Two trains collided at the Oceania subway station on Monday, leaving at least 12 people injured. (Xinhua/Jair Cabrera)

Rescuers work at the site of the collision of two subway trains in Mexico City, capital of Mexico, May 4, 2015. Two trains collided at the Oceania subway station on Monday, leaving at least 12 people injured. (Xinhua/NOTIMEX)

Rescuers work at the site of the collision of two subway trains in Mexico City, capital of Mexico, May 4, 2015. Two trains collided at the Oceania subway station on Monday, leaving at least 12 people injured. (Xinhua/NOTIMEX)

Policemen transfer passengers after the collision of two subway trains in Mexico City, capital of Mexico, on May 4, 2015. Two trains collided at the Oceania subway station on Monday, leaving at least 12 people injured. (Xinhua/NOTIMEX)

Rescuers work at the site of the collision of two subway trains in Mexico City, capital of Mexico, May 4, 2015. Two trains collided at the Oceania subway station on Monday, leaving at least 12 people injured. (Xinhua/NOTIMEX)

Firemen work at the site of the collision of two subway trains in Mexico City, capital of Mexico, May 4, 2015. Two trains collided at the Oceania subway station on Monday, leaving at least 12 people injured. (Xinhua/Jair Cabrera)