Four Boston subway workers were sent to the hospital after a collision between two Subway trains, authorities said.

Officials say both Green Line trains derailed around 9:20 p.m on Wednesday and crashed into each other.  The subway system experienced service disruptions till Thursday morning, however, by the end of the day, the service had returned to normal.

At Government Center Station, a two-car westbound train having around 20 and 25 people collided with a two-car train that had no passengers, according to a Massachusetts Bay Transportation Authority representative.

Four operators of the train were sent to the Massachusetts General Hospital and by Thursday afternoon three of them had been released, said the General Manager of MBTA. After the accident, he did not reveal the extent of their injuries, but he added that all four were able to exit the Subway trains themselves after the collision. All four employees have been put on paid administrative leave until the outcome of the inquiry, as per the standard procedure.

There were no previous safety or regulatory infractions, he said, by the driver of the train that collided with the empty train.

The root reason is still being looked into, but according to Poftak,  General Manager of MBTA,  there has been no proof of a problem with the trains, signals, or track equipment, however, human error might be one possibility.

An investigation of the MBTA’s safety has been launched by the federal government in the wake of a string of fatal and injury-inflicting incidents.

In April, a 39-year-old guy died of a faulty subway vehicle door when his arm became trapped. In September, an escalator malfunction injured nine passengers; in July, a Green Line train crashed into the back of another trolley, injuring more than two dozen people.

The National Transportation Safety Board and Federal Transit Administration have been informed of this recent crash.