The US recommended a system to prevent Green Line crashes 13 years ago. It still hasn’t been installed.

Maintenance workers carried a ladder into the Government Center Station, which was reopened later Thursday.Craig F. Walker/Globe Staff

A collision Wednesday night that sent four to the hospital was the second time in a year that two Green Line trains crashed — safety incidents that could have been prevented by a technology the federal government first recommended the MBTA install 13 years ago.

The collision at Government Center Station had some riders questioning whether the T can keep its most basic compact — moving them from place to place safely. And it comes as the Federal Transit Administration was already on the ground in Boston conducting a nearly unprecedented review of safety at the Massachusetts Bay Transportation Authority, after a series of failures including the dragging death of a Red Line passenger in April.

“The incidents are so severe, so frequent, there’s so little sign of progress or positive trajectory — in fact I think we’re seeing the opposite,” said Chris Dempsey, a former Massachusetts assistant secretary of transportation and current Democratic candidate for state auditor.

Dempsey was on the Green Line near his home in Brookline Wednesday night when the crash occurred. And for the first time in 40 years as a T rider, he said, he’s questioning the safety of the system.

The Government Center collision occurred shortly after 9:20 p.m. when a two-car, westbound E train carrying between 20 and 25 passengers struck a two-car B train that had no passengers and “was preparing to enter service,” MBTA spokesperson Joe Pesaturo said.

Both trains derailed. Three Green Line operators were taken to Massachusetts General Hospital with unspecified injuries, and a fourth operator requested medical attention about two hours later, Pesaturo said.

By 10:15 a.m. Thursday, Pesaturo said, both trains had been rerailed, and by 11:45 a.m., three of the four operators had been released from the hospital. No passengers requested medical assistance, he said.

The operator of the E train that hit the other train has been on the job for 3½ years, Pesaturo said, and does not have any safety or rules violations on his record, said MBTA general manager Steve Poftak. The T has put all four drivers on paid administrative leave, Poftak said.

The speed limit in the area is 6 miles per hour, Pesaturo said. It is not clear how fast the trains were traveling.

T riders were frustrated and confused Thursday as Government Center Station remained closed until the afternoon.

Declan Cullinane, 20, of Newburyport, had come into Boston for a day trip and said his previous times riding the T were “good experiences.”

“After hearing what happened last night, I was a little scared,” he said.

An MBTA worker offered a commuter directions from the Government Center Station, which was closed.Craig F. Walker/Globe Staff

A spokesperson for the National Transportation Safety Board said the agency is not investigating Wednesday’s crash.

Green Line riders are all too familiar with crashes. Last summer, a Green Line train traveling above the speed limit crashed into the one in front of it and injured 27 people. The driver, Owen Turner, who had a history of speed infractions, has pleaded not guilty to criminal negligence charges.

The NTSB first recommended the MBTA install a GPS-based train collision prevention system, called positive train control, in 2009 following a Green Line train collision that year that injured 49 people, and another one in 2008 that killed a train operator. The system is able to apply brakes as one train approaches another train or a stop signal, even if an operator doesn’t.

In 2019, 10 years after the NTSB’s recommendation, the MBTA awarded a contract for an alternative, less expensive safety system that it said will provide the same benefits by 2024.

In January, Poftak said the agency planned to speed up the implementation of the technology on the Green Line to 2023.

Poftak said Thursday that the technology would have prevented the crash last summer and likely would have prevented Wednesday’s crash as well. Poftak said the Green Line’s uniquely short trains and short distance between trains made finding a suitable technology challenging.

“An off-the-shelf solution that is built for a subway system, which typically has much broader headways and larger trains, doesn’t quite work in this case,” he said. “I surely wish that this had not taken this long to install.”

In the meantime, Poftak said the T is monitoring the speed of Green Line trains with fixed radars and audits using handheld radars, and disciplining operators for violations.

He said there is no evidence of any problems with the trains, track, or signals during Wednesday’s crash.

In a statement, the Local 589 Carmen’s Union, which represents T operators, said operators would like to see more “training, technology, and onboard safety personnel.”

“We will carefully review the details of this particular incident and will be communicating to MBTA officials any, and all additional recommendations for how to best avoid such incidents in the future,” the statement said.

Since the Green Line crash less than a year ago, safety issues have plagued the MBTA, including a fatality last month that prompted the extraordinary step by the FTA of conducting a safety inspection. The inspection is only the second time the agency has intervened on the local level in that way.

In September, an ascending escalator malfunctioned at the Back Bay Station and suddenly plummeted in reverse, causing a bloody pileup at the bottom. Nine people were sent to the hospital.

Two days later, a Red Line train derailed and hit the platform at Broadway Station with 47 passengers on board. No injuries were reported.

In January, a commuter rail train struck a woman’s car, killing her, when the crossing gates and flashing lights meant to keep cars off the tracks in Wilmington did not activate in time. Keolis operates the MBTA’s commuter rail system.

And in April, 39-year-old Robinson Lalin was killed when his arm became trapped between the closed doors of a Red Line train at Broadway Station and he was dragged.

In a letter on April 14 announcing the FTA was taking an “increased safety oversight role” of the T, the agency cited a “pattern of safety incidents” and said, “it remains unclear what actions the MBTA Board and executive team are implementing to prevent and address the system’s safety violations.”

A spokesperson for the FTA said Thursday that agency representatives are still collecting information as part of their inspection in Boston.

Last week, Governor Charlie Baker, chair of the MBTA board of directors Betsy Taylor, and other state officials said they still had faith in Poftak as general manager.

“I think the work that they’ve done there . . . to make the investments in modernizing the T and to deliver on a really aggressive capital plan is a huge part of creating the kind of operation and organization that can be reliably depended upon going forward,” said Baker.

John R. Ellement of the Globe staff and correspondent Matt Yan contributed to this report.